Seaver's off-speed stuff is better than most --- especially his slider --- and his fastball is adequate. While his curve as tons of break, it's a challenge to throw it for a strike at the very bottom of the zone --- too easy to throw it in the dirt. That' ok, because a lot of hitters will go fishing, even at a ball below the zone.
His biggest downside is his sinker --- it's flat. It doesn't sink. It's 2-3 mph slower than his fastball but that's the only difference between his sinker and 4-seamer.