I can't say for certain yet if Voyager as a whole is or isn't underrated. I failed to stick it out during the original run (I don't really even remember what parts I did or didn't see, though I definitely saw plenty of Seven and I remember enjoying the Hirogem stuff, but I also didn't necessarily see every episode leading up to that). I am currently working on a (re)watch, concurrently with rewatching TNG, but especially with VOY it is slow going because (halfway through season 2) I pretty much still feel like I'm waiting for the show to start and the characters to gel.
So based on my experience all I can say is: if VOY truly is 'heavily underrated', then the parts I haven't seen yet had better be absolutely mind-blowing, because what I have seen isn't getting them anywhere close.