Finnissy is underrated.
I would say under-known, like pretty much 95% of the interesting composers creating things today. Along with Finnissy, several other names are good at evoking blank stares from music students, including...
Pascal Dusapin, Hans Kox, Charles Wuorinen, Christopher Fox, Wolfgang Rihm, Sofia Gubaidulina, Galina Ustvolskaya, Rzewski and so on and so forth...
Most of these great composers are barely even "rated" at all let alone underrated. These days a good composer can't get known unless they achieve some kind of subculture notoriety, like John Zorn, whose sub-standard free jazz noodlings and dadaist nonsense reel in lots of fans and allows them to stumble over his lesser known (and far better) chamber compositions.