This, this and this.
I don't mind a sound effect here or there, but the amount of space taken up that the rest of the toy has to build around, nearly always compromising the transformation is never ever worth it.
the mech tech weapons that would not lock in position.
headmasters for the same reason as electronics boxes. theoretically they should allow for a more interesting transformation because there is no longer a head that has to be allowed for, but then there is now a cockpit that has to be allowed for and it's always much bigger than the head. on Rare occasions there is a super elegant design that absorbs the cockpit into the transformation, otherwise they get in the way of a more interesting transformation.