Mandiblasters should instead of killing stuff just prevent that model from making any attacks in the current round of combat if hit...
There is wish listings, then there is pipe dreams, then there is... what ever this is. That is, without a doubt, the single most broken suggestion I have ever heard. I know of one ability in the game that prevents a unit from attacking in combat, and it only moves their ability to activate to the very last unit chosen.
Full on preventing models from attacking? No, that is going way too far.
Well, that would be broken if it were on a unit basis, my suggestion was on a model basis, only models hit by the mandiblasters would be unable to fight for that assaultphase. The mandiblasters would still need to hit, most likely on a 3+ so a unit of 10 scorps assaulting 10 marines, may have 6 marines in range and statistically 4 of them would be stunned by the mandiblasters, so still 6 marines hitting back. In addition the marine player can opt to choose stunned models as the first casualties.
You also wouldn't be able to use the mandiblasters in the opponents round.
This can of course be effective to numb characters in CC or simply to lock down units.
Scorps current 2 S4 attacks are not that scary for an elite CC unit and eldar needs one living CC unit that can stay alive for a while.
Sure you could water this down to having a -1 to hit them in CC, but then the mandiblasters should hit automatically.