Some of it can be helped by unlocking the "Burning Rune" (Eldar "Iron Halo") which is found at Rank 5 at the end of the "Generalist Tree" (Plasma Grenades, Medikits, Fusion Pistol Mastery and an useless standard, ending with the Burning Rune). It is a rather costly way to go, but the Grenades and Medikit unlocks are somewhat required so that helps a bit.

The "Advanced Aspect Armour" costs 85 AP to unlock and in case you are interested, then you need to unlock the following amount of stuff before you can unlock the "Advanced Aspect Armour".
Warlock: 4 Advancements (Armour is the 5th item)
SS, DA, SH, HB: 6 Advancements (Armour is the 7th item)
FD, DR: 5 Advancements (Armour is the 6th item)

The Burning Rune is placed after 7 Advancements and costs by itself 150 AP (the useless standard just before it costs 100 AP).

In any case, Eldar are imo less "viable" before they get into high rank, as they have access to less gear. It does help a lot that they can unlock stamina gear as fast as the other factions, so they aren't really far behind. The Dark Reaper is a bit worse though, as they (like all other heavies) doesn't really benefit a lot from stamina.
Orkz, by comparison, seems to be rather good from the end of rank 2, as the high HP Wargear really helps them a lot.

Burning Rune:
Wargear Slot 1
Cost: 400 LP
Benefit: 40 Armour
So just like an Iron Halo, but a bit harder to obtain.

In any case it's, from a lore perspective, okay that Eldar have a harder time accessing "direct" survivability gear, but it would be nice if they could get something "Eldar" instead or if the different Aspects would progress differently.
Striking Scorpions, Dark Reapers and Fire Dragons are all known to carry surprisingly resilient armour, but the only proof of that ingame is that they start with 100 toughness and that FD and DR have access to a somewhat inefficient +20 Toughness Wargear (previously the Striking Scorpion could get this one too).
Something "unique" like the Howling Banshee's wargear for all Aspects would be awesome, and hopefully that will come Soon(TM).

It might be a best value for point, but its still worst "gold tier" armor in game by far.

Most pointless thing is, each class have its own aspect armor, but that aspect armor have mirrored stats and costs to everything else, the trees and armor placements seem like its going to be something as specialized and class-tailored as it is with LSM halos for each class, but as it is now, all the aspect armors could be locked behind 2 upgrades, 1 for light classes 1 for heavy classes if the stats are to be the same. Plus, Eldar are much more limited by not having "red tier" armor, you either are forced to overpay for survivability or have no survivability at all(anyone who played Eldar knows how nightmarish it is to use presets and battleloudouts before R4).

That is a negative. Eldar just start with less stats to offset their speed, acceleration, and hit box size bonuses. The armor itself is superior, albeit slightly.

Unless its someones first FPS ever and they have really casual, slow reflexes, speed isn't really relevant for survivability. Vaulting speed advantage saved me multiple times, run speed never.

And if you add slightly more stats to inferior baseline, you'll still have less stats at the end.
Eldar start with 20 less toughness and 20 less armor or 30 less health depending on faction-armor does not make up for that difference at all, especially with other factions actually having weapons, not only melee but also ranged that completely nullify that toughness eldar get.

Again, point for point it might be superior, but on the context of all base stats and what all factions have, its still inferior.

Are you suggesting that, for the same cost, Eldar armor should provide additional armor to make up for the other factions' baseline stats? Because that sounds like a rather non-objective point of view, not to mention unbalanced. In addition to their faction bonuses, they should also get another faction's bonus on top of that?

No.
I'm saying for everyone to stop acting like its best armor in game, because end stats are still on much lower level then anyone else and while its still a great boost for eldar classes, other factions have much more options here because of existence of cheap "red tier" armors alone.

I agree that Eldar would massively benefit from a middle-tier armor like all the other factions have. Having to devote nearly half of your load out points to the only armor available really sucks. Deliberate design decision, or GW restriction? Because it's too big to just be an unintended oversight.

However, objectively speaking, their gold armor is the best. In the same breath, objectively speaking, Eldar have the worst base stats in the game. Therefore, those two facts allow the situation to come about where even with the best armor, it won't overcome the base stat differences. Does that make the armor inferior? No, it makes the base stats inferior. Accept that and move on.

Only the Dark Reaper really suffers from the lack of a mid-tier armour, as the 400 LP leaves them with little to no room to equip anything else if they use their signature weapon (Reaper Launcher, 600 LP without Mastery). If they equip the Armour, a Reaper Launcher, they have 100 LP to use.
If they want to use the Tempest Launcher (most overpriced weapon ingame) with a cost of 680 LP without Mastery, then they will have 20 LP to used on other stuff.

But in any case, this is only a real problem with the Tempest Launcher, and the problem there is the weapon itself, not the Dark Reaper or their Wargear.

So basically what you're saying is MoS is worth even less than the value ascribed to it because it only affects the non-important run speed?

MoS and other marks do not affect opponents or their accuracy in any way.

They all cater to different playstyles, to me personally, MoS is most useful on JPA(obvious reasons), GA with lifesteal mod(so every single one) and sorc who goes for slaanesh powers.

Using MoS, being in places where you can be seen and shot at is about the worst thing that can happen to you, because you have no sustain the armor would grant you otherwise unless you are self healing sorc.

MoS is ambushers and flankers mark, you should never engage directly and frontally using MoS, you always should try to find and go for opponets blind spot as additional speed makes that easier, you're also going to run down non eldar runners with it, but in direct combat MoS does nothing to you, unless opponent is shooter cripple of "bronze or worse" level.

Its not really hard to aim at eldar, especially their heads, which are rather easy to aim at because of the helmets with different colors, basically telling you where to shoot.

MoK and MoN are combat marks, both will keep you longer in direct fights, one works better for ranged, one for melee, but they can be used according to preference.
MoT is hit and run mark, fast armor regen can and should be abused.
MoS is flanker/ambusher mark, you're at biggest disadvantage in direct combat, but with MoS you need to pick best targets of opportunity.

MoS only affects how quickly you'll get to the point/opponent, it gives you absolutely least survivability out of three marks. Its great mark, don't get me wrong, but just like eldar speed, MoS speed is overrated and won't win you any direct fight.