[The Might]

Well, she does seem to resemble the Balrogs in a way. Joins Melkor, comes down on Arda and then assumes a certain shape which she can afterwards no longer change.

Actually the whole idea about Tom Bombadil isn't that bad.
Because, if we think she wasn't an Ainu, then she could just as well be a discord in the music, something in M-e, but not really of M-e like Tom Bombadil.

And, very interestingly, in early writings Ungoliant is reffered to as Múru, which in Quenya would mean Primeval Night.
This does lead to some speculation that she was in a way also a spirit of nature, however a dark and evil one, whereas Tom Bombadil and Goldberry are positive ones.