I'd say practically the whole library is underrated. Too many gamers think there's absolutely nothing worthwhile on the PC-FX (the sorts who have probably never even played a single PC-FX game; they're the ones who say the PC-FX has nothing but hentai games too), and then you got the ones who think there's nothing worthwhile beyond Zenki, Zeroigar, and Chip-chan Kick. I can't blame Western gamers for latching onto the English-friendly games that are in familiar genres, but there are other very English-friendly PC-FX games and even with some of the games that do have a fair amount of Japanese text, some are very easy to figure out with just a little bit of patience.
I own well over half of the PC-FX library, and I'd say the quality to crap ratio on the system is very good. Very few PC-FX games have struck me as poor games. Granted, I've yet to play a PC-FX game that I'd say ranks among the very best games I've ever played either, but I don't expect every game to be the best game ever. The PC-FX has provided me with many fun, interesting experiences, like with Farland Story FX, Last Imperial Prince, and Welcome to Pia Carrot, and I wish all those games and many more would get a little more respect and love from the Western retro gaming populace.