They are probably the first band that comes to mind when I think of "underrated".
Their biggest problem was their inconsistency. Their insistence on Gil Moore singing really hurt them, not to say that he didn't sing a bunch of awesome tunes. But give him maybe two vocals on an album instead of half.
I mean...it worked out pretty well for Styx. Rik's songs were all top notch. If there was a dud, it was guaranteed to be a Gil track, but like you said, he also had some great ones. And I definitely agree that they're underrated. Emmett only showing up on a few lists in the recent singers countdown was disgraceful.