It's not necessarily that it's bad, though the potency disparity is nothing to sneeze at if you look at the compiled logs on various fansites. We can do the job, but our window for error is so minuscule that one single mistake can cost the party dearly. SCH has always been the class to play if you're the kind of person who memorizes fights, and in that capacity some of their old usefulness does still continue to shine. However we cannot main heal to save our lives, so while we can keep everyone /up/ that doesn't necessarily mean it'll always be at full health. Even then however the reward for learning the best times to pop skills is negligible compared to what the other healers can experience. Leading many SCH main's to wonder why it's even worth it to put in three times the effort for lesser returns.
With that said however, what WHM still lags very far behind AST in sheer terms of versatility and utility in spite of far outstripping SCH at the moment. So while it's in a /better/ spot, it still needs adjustments for optimal balance with AST.