Because FFVII certanly is.
Ever since the game came out, the fans and haters have always been very strong with there opinions and many refused to even hear the other story. As time went by there where multiple debates, on FFVII.
A hater of something will always notice praise on it and will never understand why anyone could like it. Likewise, an obsessive of something will be very upset to see anyone express hate over something so good. This is where flame wars come into it, and POVs and opinions become exagerated. Games like FFVII will always seam underated to its biggest fans, and overated to its biggest haters. A good example is when it was reviewed in PLAY (UK Playstation magazine) it was awarded 93% - one of the top scores (though not what I would have given it). However, the readers of PLAY had mixed reactions, and every so often a letter of complaint was sent to them; 93% was concidered too low a score by the fans - too high by the haters, such was the devided opnions of FFVII
So basically I think it's just a game you either like or dislike. I personally love it, and rate it as one the most epic games of all time. So to me, it is underated, but only because as a fan I notice the negativity surrounding it more. But that's just me.