Buu wasn't even THAT strong, he was "defeated" time after time after time JUST like cell, but the blasted regen tenacity of them made them kind of a threat.
BRB survived kaiokenx20 kamehameha
BRB survived direct hit from spirit bomb
BRB survived entire planet exploding while being cut in half from waist down
BRB No regeneration at all.
BRB Was literally a balling CEO of a planet trade company