Personally, I think one piece that is either very difficult to use or almost useless is the pirakha right arms. I have tried to use them in the past, but nowadays it seems near impossible to use them without building some sort of rig around the part. Although, it can be pretty decent as armour on slightly larger MOCs.
@Ekorak Although the black bohrok head things are decently useful for legs on a toa-sized MOC. But they are neigh useless as arms barring a titan MOC.
Edited for Double Posts