Near as I can tell, the carbines break down like this:
Best at medium to long range: AK 5C. Low spread and recoil, highest muzzle velocity and range, but low damage. (oddly, not the lowest damage carbine, given the advantages listed - gun is OP for sure) This is BF3's ACW-R. You can pretty much just use this forever if you want.
Best in pure close quarters, where such a thing exists: ACE 53 CQB. Best TTK, but poor range and high recoil. BF3's MTAR. I'd rather sneak around with a shotgun, but it could be fun in domination on the right map.
Happy Medium: ACW-R or maybe M4. Better damage vs slightly better control and range. Giving up a lot over the 5C for a bit of damage, though.
At least there is no weapon that is the best in 90% of all situations this time like the M4A1 was, but I don't see any reason to use any of the carbines aside from the ones listed.
The one thing I haven't factored into this is the effectiveness of burst mode on the AKU-12. I haven't tired it out yet because I'm sure it takes practice. 1000rpm 4 round bursts could be pretty powerful. I'm sure there will be faggots with repeater macros owning up with it.