even with current amount of 1 hitbox (a VERY low definition model huh?) servers are pushed to the maximum (64 slots, 999 bp etc.) with anything consisting of more than 6 faces to check the CPU load will go up a lot.
On the other hand, a WolfET mod : jaymod, has introduced an oriented hitboxes, and also had about 10 of them per player, moreover, they have moved, following the player animation, such as walking. But even that did increase the load a lot.