As far as the stealth mechanic of the Striking Scorpion in EC the way it is implemented now is completely worthless. Yes it allows you to "sneak" around almost undetected if you use shadows, walls, barriers and move slowly, but so can every other class. I have more success "sneaking" or flanking opponents with a Fire Dragon and melting them with one shot from behind than I do with a Striking Scorpion simply because I can move faster and once behind your enemy they can't see you stealthed or not. In order for Scorpions to remain stealthed effectively they have to move to slowly to keep up with most enemies movements on the battlefield.
Warlocks are better in melee than Scorpions and once the Howling Banshees are released I see no reason to play Scorpions at all.
If stealth attacks had a stun or some kind of damage multiplier then they might be worth it or if they had the ability to capture points then maybe they'd serve a purpose. Right now however they are just to slow to really be much of an impact.
All "stealth" really gives you right now is one alpha strike that does the same damage as any other attack and like I said if you really want to sneak up on someone and kill them in one shot just use a Fire Dragon and move using cover to get behind them and melt them into nothingness.
The Stealth mechanic as it currently works is just a gimmic that's interesting to see but serves little purpose.

The Warlock would have 50% if Conceal was the only bonus they had.
They could raise it to 95% by Standing Still, Crouching, being in Darkness or taking Cover.
Adding another of those modifiers(I.E. Conceal, Crouch, Cover) would be 100% Stealth.

but if that's the case does everyone have invisibility or only SS and whoever happens to have the conceal bonus? because the way you're making it sound is either
A. people who have cloak from any source gets benefit from the multipliers such as standing still or shadows ect.
B. Everyone has cloak but SS and Warlock do it better

Me being dumb, I am slightly confused.

imagine stealth with a power fist.

yes eldar have power fists

yes our power fists have guns on them.

that'll be fun lol

Honestly the stealth mechanic works really well as-is. I think the biggest problem with it is that people don't understand it.

Crouching down and slowly walking is the absolute worst way to use it. Other than the fact that you're out of the fight for a long time, it makes you REALLY easy to spot. Anyone is able to see a slow moving blob going across their field of view.

The best way to use it is to sprint. Just run behind cover, crouch down for a second to get full concealment, sprint to some other cover and repeat. You can cross large areas very quickly and dart so quickly out of people's field of view that they don't see you at all. The concealment actually works really well when using it that way. You can easily get to places that other classes can't, as you can easily move around to where one of the other brightly colored classes is easily spotted.

Coupled with the fact that you're pretty much invisible while stationary you can pull off a lot of good tricks. One of my favorites is to interrupt a cap. You sprint to the point, quickly crouch and then reset the cap. People can be standing right next to you and not even notice you. It's kind of funny. It's also great for resupplying or to rez someone without being seen.

The concealment also works really well for escape. If I get surrounded after a quick assassination, I jump off the first ledge I can find and immediately crouch. IF the enemy team even tries to come after me, they often run right by me. I then to the same thing over again. It can really mess with the other team. Once you get used to moving while cloaked, you see how useful it really is.

Honestly, if there's one thing that REALLY needs to be fixed it's the scorpion's piss-poor damage output. Sure, a charged attack to the back will kill someone, but in a straight up fight, the scorpion is pretty terrible. Needing 5-6 hits to take down a marine who's not even trying to defend himself is pretty terrible.

Back to stealth though, crouch walking is bad. Sprinting while stealthed works perfectly well once you get used to it and then you're not a slow waste for your team.

It sucks because you can be spotted hundreds of meters away. I'm told this is a bug.

Only the SS and Warlock would be able to self-Stealth. The Warlock, via their Stealth mechanic(Conceal) would be able to provide Stealth for allies.
It's A, with Scorpions only being able to "cloak" themselves, while Warlocks can Stealth allies in an area around them.

Being able to crouch for a second to get full concealment and then sprint is exactly why the current system is bad. Performing actions like sprinting should at least partially reveal you as no single action should provide you with 100% Stealth. It should be a combination of factors, which in turn allows for means to be used to break your Stealth, without illogical means such as character model or reticle proximity.

Things like being Still, Crouching, being in Darkness or even taking Cover should provide a 45% Stealth bonus, meaning if you had at least 3 of those modifiers active you would have 100% Stealth.
The Scorpion should have an Active Stealth ability that provides them with 50% Stealth so long as it is in use, while the Warlock would be able to provide 50% Stealth to themselves and any nearby Eldar via their Conceal ability.

With such a system, you could sprint to a point, crouch near it and reset it. After jumping off a ledge you could crouch and use your Active Stealth to gain 100% Stealth so even if the enemy team ties to come after you, they will often run right by you.

Scorpions should deal less damage per attack than an Assault Marine and have a similar attack speed so off just melee weapons the Scorpion is at a slight disadvantage; however, the Mandiblasters should add to the Scorpion's offense and even the scales.
For example:
SM Chainsword TTK vs Scorpion: 2.1-sec, 7 Basic Attacks, 0.3-sec Attack Speed
Scorpion Chainsword TTK vs SM: 2.4-sec, 8 BAs, 0.3-sec AS
The Mandiblasters would do a bit less damage than each Chainsword attack, could have a 1-sec or so cooldown and could be activated in various ways such as: Charge Attack, D.Bash, Block(which should replace clang).

Additionally, because the Scorpion Chainsword has AP6 vs the Imperial Chainsword's AP-, the Scorpion should be able to deal 16.7% of their damage through the SM's Block.

If you really need to move at such a pace that you cannot maintain 100% Stealth then you should be relying more on your teammates to keep your enemy distracted. You should be crouch walking as being crouched should be one of the ways to reach 100% Stealth. As being Still should be another Stealth modifier, performing actions like moving/sprinting should remove those modifiers, reducing your Stealth level.

This is also how I use it and I find the actual stealth mechanic itself is great. I agree with you entirely that what isn't great, however, is the Scorpion's ineffectuality when it comes to actually fighting. The Mandiblasters are going to have to be seriously great to give them any balance.

Edit: Although I should say that in my experience I can't one-shot enemies when I hit them from behind with a Heavy attack. Don't know if I've mostly been trying it on folk with a MoN or what, but I had matches against Space Marines where I still didn't down Apothecaries and so on despite giving them a solid Heavy hit to the back.

The problem with Striking Scorpions, IMO, is their slow paced gameplay. You have to crawl behind an enemy and then charge your heavy strike if you want to oneshot your target. Hoping he doesn't decide to move. That mean SS is only viable against static target like devastator/havoc or player interacting with pointcapture/medikit/other players. Also you are probably able to put down an enemy but, the effort you put in this, leave you open to the attack of his comrades that invariably will eliminate you and help your unfortunate victim (but maybe i'm very bad in melee).

So, in conclusion, i'm perfectly fine with how the cloaking system works. You have to be smart in your movements, decide when is better to run or creep and use shadows while hiding. But i think that SS should pose a greater threat while attacking. A damage buff that start when he's cloaked and fade away in 3-4 seconds when uncloaked could be an idea.

Also in the next video there's an awesome guide on the topic: "How not to be seen"
I suggest everyone to watch it.

Overall, I'm happy with how the Stealth works for the Scorpions but I feel very hamstrung with the general speed of movement for them.

Also, setting up that sweet assassination move from behind to pummel a marine is very satisfying. However, once you are revealed and need to get a move on to not get repercussions from the dead marines buddies. It feels like I don't have the movement to do it. If I evade, I stop after flip for a half sec (which results in a chainsword to the head). Maybe I'm just channeling a Assassin's Creed feeling.

That being said I do feel, to make the class unique/workable there should be some bonus to a successful cloak-hide-hit. Maybe some sort of stun if the devs think a stealth-strike bonus is unfair/unworkable.
I can see in future the Scorps getting a "Hidden Strike" power. Maybe Mandiblasters providing a Stun/Damage Bonus.

The main characteristics I associate with Striking Scorpions:

• Stealth: Check
• Same overall speed as SM: Check-ish (Full cloaked I think should have a faster movement, not the same as running around but a bit faster just from playability view point). Stopping after a flip evade just doesn't make sense.
• Chainsword gives same strength as SM: doesn't seem to be there in the game. SS attacks seem weak.
• Mandiblasters: not there. In the table top game, older editions gave a bonus attack, recent edition is a automatic hit (D3 hits I think). In the fluff, it was more like a boxer's jab setting up the next hit combo. My guess is Mandiblasters will show up as an Execution animation which would be a bit disappointing to be honest.

One other thing about the Scorps that bothers is the Chainsword sound effect. I always imagined it was closer to a cat purring than a chainsaw. From the BL novels that's the impression I got. A quieter, more elegant weapon.
Not sure if my target can hear it when I charging up the heavy hit but it is bothersome to think that they can when I'm meant to be all sneaky and s**t.

I've yet to play the other factions yet (Self-professed Eldar Fanboy) but I would hope a Raven Guard marine or a Alpha Legion CSM would be able to do some sneaky stuff too, to fit their backgrounds.