i have multiboxed hunters, mages and paladins in the MC raid i organise for the guild.
I have also had 3 hunters in BWL in guildruns multiple times.
Onyxia i have done both paladins and hunters and the hunters simply require a healer to keep them up as its simpler to stack and get a healer to heal them (or as you have mage+priest having the priest heal the mages should be just fine).

As already have been said, where you cant stack to much or need to move it can be a bit of a hassle.
But in MC that is on the bomb for example to pick the correct window.
Or on raggy when you get blasted.

These things can be countered and there is no real issue if you stick to same class.
Because if there isnt a free spot for a useless priest, the raid would want the priest to also heal the raid.

Just go for it, its easy peasy in MC.