I dunno if that was really underrated, so much as it was just not released in america. that game was actually very big in japan. I totally agree that that game is great though.
I only know of it because I my cousins had illusion of gaia when I was young. later on in life say mid high school I got into emulation quite a bit, even modded a forum for a site about snes emulation (which in my opinion it was the best emu site for a while before nintendo forced the owner of the site to stop). either way I mentioned illusion of gaia to people there and everyone right away told me to play terranigma.
though if we are talking about games that never made it to america that were awesome, I loved 'treasure of the rudras'. I haven't even beaten that game, but I was like 2 or 3 bosses away from it though, it was such a super long game too.