Kid Buu has nearly everything. Speed, long ranged normals, beams, reliable down+H, mystic ball that is safe on block and can even confuse enemy about his whereabouts, ability to punish enemy from long distance regardless if he is on the ground or airborne, high damage. His only weakness are supers which arent that great when used raw but as meta matures supers are less and less used outside of the combos so it will be smaller flaw in future (because its not big one even now).
Kid Buu's strings are pretty linear, so learning how to properly block and punish them will be the most helpful, since there are very few different things a Kid Buu can do. That being said, not blocking and punishing properly will absolutely decimate your team, so be cautious when fighting Kid Buu.