I think it best to steer away from the "undeservedly sung." Besides, didn't we already have a thread on that a couple of months ago? It was actually titled something like "music I just don't get," but don't we often have a tendency to devalue that which we don't understand? I can think of many, many composers who I didn't "get" earlier in my life (or even fairly recently), and whose music I've subsequently come to love and admire.
My prime nomination for "deservedly unsung" would have to be Roy Harris' 13th Symphony. Premiered in 1976 by the National Symphony, it lay unheard for decades. I wondered why, once I learned of its existence, since I value much of Harris' other output highly. Well, once I heard a recent revival on YouTube (and I think the work was posted on our Downloads as well), I fully understood why. This is an embarassment of a composition from any composer, let alone a major one such as Harris! Trite, banal, simplistic, shallow, pretentious, are all adjectives that come to mind.
Mind you, there's VERY little music where I don't find something to like about it. I usually find some redeeming quality in most anything I hear. But this work is not even remotely approaching tolerable!