They also have the hardest and probably most bullshit questline.Guardsmen are pretty weak. Ratlings and Chimeras (Chimerae?) help a lot with the early game. IG are mostly a vehicle faction once you get past the early game.
I have not played Necrons but I think their armor and regeneration makes them relatively easy compared to IG.
After you do the third (iirc) mission you're pretty much stuck fighting waves of enemies two missions in a row, and the last mission wants you to kill something like 20 kastellans and a techpriest within 20 turns.
Necrons, Orks and Space Marines at least have the decency to have you choose to activate a relic before starting the final mission. You don't get that with IG. It's very much possible to lose the game with them by accident if you do their questline.