One thing that has really bothered me about the melee stun combos in MP, is that they seem pretty worthless due to the non-effect is has on enemy players. Typically I play a Tactical Marine, and I use the serrated knife perk because I like getting in close.
The problem with stuns is that they leave you vulnerable to your target and enemy players. I don't know if it's a bug, but when I use a stun combo, the enemy can just keep on moving and attacking, while I'm stuck recovering from the stun animation - so it seems to have the reverse effect you'd hope it to have. So what's the point of a stun in MP? Is this a bug, or does it have to be improved?
Oh, and nothing is more cheesy and frustrating than an ASM/Raptor with Final Vengeance and getting stuck in a stun attack animation after you kill him.