The UCLA abutment was designed to fix implant angulation and certain depth problems. Since it is fully castable you could do everything with it, a direct crown or a personalized abutment for a cemented crown or bridge, a dolder type bar. Even ball attachments are made to an angle with this abutments.
The only flaw of the fully castable UCLAs is the implant abutment precision adjustement gap.
Because of that, casted abutment can't compete with a machined abutment.
That is why the overcast UCLA was invented. It has a metal base (Ti or CoCr ) with a plastic sleeve that provides the flexibility of doing almost everything with the precision of titanium abutment.