U-smash is actually extremely punishable on reaction. It's matchup dependent I guess, but it's a very unsafe move (unless it's blocked). 11 frame charge release (which is reactable) and about 22 frames of ending lag. Not that much ending lag, but when you consider how many moves outspace U-smash, it's really not something I like throwing out too much.
U-smash is actually okay in the Jiggs matchup since a shielded Pound can be Up-smashed out of shield (although usually they're going to use Pound in reaction to you doing a move or to punish landings). I wouldn't U-smash against Wario though, since reverse U-air is superior for most purposes except grab release combos. And I definitely wouldn't U-smash against someone like Marth or G&W.
And I guess it's rewarding when you land it in D-air combos, since it sets up juggles alright.
And it's kinda odd to be saying this, but I really think Ganons should use more Dash Attack. It's really an amazing move. The threat of it alone is enough to get people to play differently (which usually means shielding or retreat spacing when they see you running at them). Because of how amazing Ganon's DA is, running at people with Ganon I believe is his best approach.