Balrogs as Maia are of the same order of beings as Gandalf, and indeed Sauron himself. Nazgul well they have a devolved power and some major weaknesses eg being water funks. Glorfindel is more than a match for half of them together and he isn't a Maia.... though quite possibly the greatest of the Eldar would equal or exceed some Maia in inherent power. The main weapon of the Nazgul is fear...and surprise..and a fanatical devotion to Sauron of course
