You'd be surprised; many players say he's just outright bad. I think it's just a social dogma pertaining to how horribly misused and ineffective he is with bottom-ladder players.I wouldn't say he's underrated; I think that most people know how good he can be.
The problem I've always had with Scarftar is that Weavile is just plain faster and stronger. Granted, Weavile doesn't get that awesome physical or special bulk associated with it (though his ability to take a special attack or two is surprisingly decent), but Scarftar has similar problems with Scizor (Pursuit gives him a free U-turn!!!) and is less of a team player considering the Sandstorm summoning makes fast and frail LO attackers like Infernape much less viable for a teamslot.
I recognize some of the discrepancies between Weavile and top OUs regarding defensive capabilities, sweeping ability, and Stealth Rock, but I don't subscribe to the popular mindset that he's along the ranks of Electivire. Electivire competes (not to mention loses) against other mixed sweepers who don't rely on a predicted boost and are much harder to wall, while nothing can really stake claim to Weavile's niche.