You won't have to be in a tank or sentinel to be effective as an IG. You will certainly die quicker than a Space Marine but an IG with a power sword will hit just as hard as a SM with a power sword, same goes for an IG with a lascannon, same goes for an IG with a flame thrower or a frag grenade or a sniper rifle.

In fact due to their squishyness the best way to balance the IG would be to give them their weapons and upgrades at lower resource costs.

Let's also not forget that although the SMs will be paying resources to field terminators, IG will have Assassins, ratling snipers and Ogryns.

Even a terminator won't stand his ground when an Ogryn smacks him in the head.

Then don't play IG. Dependence on vehicle support is part of their theme as a faction. If you don't like it you shouldn't expect to like playing as them.

They didn't say the game would be imbalanced, but rather that it will be asymmetrically balanced. As in factions/classes will have different strengths but the factions would be comparatively balanced overall.

A normal guardsman is far weaker and should not hit as hard with melee weapons as space marines.

The aiming system will have factors determining accuracy and the like so guardsmen will likely be worse at shooting than space marines.

The imperial guard do not have assassins, that was only Dawn of war. The inquisition has access to assassin temples and can be assign to help any imperial faction.

A terminator can stand his ground vs an orgyn, his armor can easily take a hit and retaliate with a power fist.

Even with lowered weapon costs most guardsmen should still be going into battle with weapons and armor worse than everyone else so they will have to have better support buffs like faster respawn, or make more of their stuff better than it is to overcome the ork f2p horde

I can't really see a way to make IG viable without overly buffing them or vehicles, and neither seem fun

Oh don't worry, we're probably going to have to nerf their vehicles. They're pretty strong.

Veterans, which are likely to be the standard "Guardsmen" of the game (based on precedent from DoW/2, as well as the TT concept of a Mechanized/Armored list), have the same Ballistic Skill as Space Marines, so accuracy won't be an issue outside of player skill. We're not going to be playing as Conscripts here.

I can definitely see faster respawn, though we've also been talking about giving them a dynamic buff system in the form of Orders.

An explanation for non-TT players:

In the Warhammer 40k game, command squads (Company Command and Platoon Command) can boost the effectiveness of infantry near them by issuing Orders. These Orders can increase their effective DPS (First Rank Fire, Fire On My Target, Bring It Down), make them run faster (Move! Move! Move!), or give them a powerful defensive buff at the expense of DPS and mobility (Incoming!).

Some special Officer units have their own unique Orders with unique effects, like Al Rahem's "Like the Wind!" (lets you shoot and run in the same turn).

The Imperial Guard also has lesser Leader Units, like the Commissar and Priest, that passively buff squads they are attached to, as well as support units like the Medic and Vox Caster that provide smaller but no less important buffs.

Skilled use of Orders, Leaders, and support units can turn the tide of battle, even when facing a statistically stronger enemy. By allowing Officer units to offer similar buffs, Guardsmen could be made far stronger than their base stats imply in a way that is immersive, consistent with TT, and consistent with Lore.

As a bonus, Orders would make the Imperial Guard appear to be more organized than the Orks, because the buffs they get can be used as zerg herding tools by making certain courses of action naturally appealing.

These are the same Terminators that get stepped on by Titans and survive, along with going toe to claw with genestealers, right...?

There are no Titans in 40k table top and Terminators come in 1 inch square bases of 5 pinhead sized models in Titan Legions and certainly do not survive even a Warhound Titan stepping on them.

Fan fiction shouldn't be part of the rules of the game.

In the table top an Ogryn can easily take out multiple genestealers before he dies (Ogryn armour will be shredded by stealer claws but Ogryns have multiple would and Terminators only one).

A Terminator with a power weapon does 1 wound, with a chain/power fist multiple would so an Ogryn vs a Terminator is pretty even, the Terminator gets a save but will be forced to make multiple saves the Ogryn will soak up the wounds without an armour save.

I will like to point out that although the game will be based off the lore, we cannot base balance purely on it.
Just reading through some of the posts on this single thread I've found suggestions that'll make SMs unstoppable.

Exactly, also a lot of the lore is based on fan fiction stories written for dramatic effect, I remember a story where a standard marine killed a carnifex by jamming a krak grenade in it's mouth, I hope people won't be shouting for carnifexes to die to single krak grenades because it happened in a story.

Lore has some realistic elements, put a grenade in a carnifexes mouth it will go bye bye, lore puts down the capabilities of how strong it all is and then puts it down in a realistic sort of environment to a certain degree.

Copy pasting lore in game piece by piece would make the SM and CSM i believe have a certain max capacity of how many can roll that race/Faction.

One will have to be wary of lore and do a bit bending, IG as some have mentioned rely greatly on the mechanized combat as well as keeping up the orders/moral it has to be structured so a lot of buffs for different things, would be pretty damn great for IG.

If you decide to stray alone and then get raped easily then your playing the faction wrong.

This only stands for hand held weapons like a sword or chain sword where you are swinging it with muscle. Once you get to things like power swords or power claws the main damage is done by the power charge not the swing of the blade, with a powerfist it's mounted on hydrolics and the flesh wielding it is just a guidance system, also there are plenty of pistols (think laspistol, metlspistol, plasmapistol ect) that also count a close combat weapons as well as short range weapons on the table top. These only require you to be able to pull a trigger at point blank range and dodge your enemies attacks.