I don't think it's underrated. It was well-received went it came out and is still highly regarded. When and how was it underrated?
I will say this (and I share this opinion) after the years have passed, perhaps the flaws and problems are more noticable.
TUC was a hit with fans, maybe even moreso coming off of TFF. It was a fine farewell to TOS. Maybe we were too willing to overlook the problems at the time, too willing to let the flaws go unmentioned.
But decades later, as much as I like TUC, yeah... there are things that bother me now that really didn't at the time.