The trouble with box wonder weapons is that they are totally random. In a 4 player game, it is hard for one particular player to get the weapon. The box could give the weapon to a less skilled player who would keep it but wouldn't use it efficiently.
Buildables are annoying though. I hate the fact that one person can run off with a part and trade it for another part or lose it. It is annoying looking for a lost part, not being able to find it and therefore not being able to build the weapon.
It is also annoying when one player builds the weapon and another player holds onto a part, then they swoop in, place it and steal the weapon.
To be honest I prefer buildable weapons because they are less random to get. I support them if the buildable system is like Mob of the Dead, where there is a HUD. I also want them to be pack a punch able unlike in black ops 2.