Surprised at the negative reaction to her on this thread. She has a phenomenal voice and great songs. She has deep and thought provoking songs, which as a result can also be sad/depressing, but those are usually better. Who actually wants to hear "happy music"? Adele is probably the best singer performing today (name one that is better) and is actually competitive with great singers from prior (stronger) eras as well.
As far as combining great voices with bad songs, a much more apt example would be Mariah Carey. She has all the range, etc. but except for her 2005 album most of her career was built around annoying songs.