It's weird to say the only issue I found with HAWKEN was cosmetic but it is.
Weapons feel like peashooters. Like seriously it's CoD level of peashooting. They are loving mechs I want to see some awesome boom bass and reaction dammit. It feels odd to play.
Based on the scaling of the mechs, though, the weaponry is appropriate for the size. Sure, the sounds maybe aren't quite as heavy-hitting as they should be, but overall it's more realistic than other mech games I've seen (in terms of reasonable weaponry, that is).
EDIT: I didn't have time to say this before, but I do agree with you, to a point. The sounds could be better. The reactions could be improved (maybe knockback; a little?); but overall, I believe it is pretty close to realism. I'm not sure if that's what you'd prefer, but that's how I see it.
Also, everyone needs to keep in mind: this game is still very much in beta. Things are liable to changes; improvements will be made. Nothing is concrete yet.