Doppelgangers only have very limited powers really...the polymorph equivalent but their stats do not change based on form. They have advantage and a big surprise hit if they can catch you unaware. They have no spellcasting capabilities, and no proficiencies in wielding any weapons or armor...much less so efficiently. They have mind reading which gives the advantage on Wisdom and Charisma checks so advantage against a bunch of magic. So they literally can't mimic any powers or wield any equipment that the creatures they mimic could, etc. They can't even cast illusions to make it seem like they are casting or wielding anything.
