The engine limitations are that it can’t do what people want out of a shape shifter class. 
If I’m understanding what it is people want out of a shapeshifter class, the engine can’t support a character morphing into some other thing, i.e. changing of skill trees, form etc.
As mentioned before, this has been asked for before and the devs have stated it wont happen because of engine limitations.
See: A shapeshifter?