Hard to sing because they are "not well written"? A guess a handful of Tony Awards and a billion dollars in profits (not to mention from countless worldwide productions) are a display of truly poor to mediocre writing. Then again, if his work were not so successful world-wide, I am sure the criticisms would be a lot less frequent. The masses buy his work therefore he must be not-all-that-great.
Just because he writes the score first does not mean there is no collaboration whatsoever once the lyricist steps in. How that process works is a question for Tim Rice, Don Black, David Zippel and Richard Stilgoe who have all attached their names to his work and did quite well in the process. But in the end, ALW is not the lyricist, yet is targeted with the blame when a clunker lands. By that rationale, the lyricist is equally responsible for a clunker of a melody?
I am not a fan of everything the man has written. Some of it has been godawful. But any composer who is that prolific is bound to hit a bump or two. He sometimes fails but at least he is still at it and occasionally successful.
I adore Sondheim but find a few of his scores (Pacific Overtures, Bounce and the better part of Passion) pretentious and dull. But I would never consider him a weak composer just because not everything lands.