You can not predict what the dynamic of a match is going to be when a stage element comes into play. When smashville platform comes by, you don't know where you are going to be. However, if you happen (by pure random luck, and nothing else) to be recovering. congratulations, the game just helped you recover. It's exactly the same as if an item happened to spawn above you that helped you recover. This is pure random luck, and random luck only benefits the worse player (because the better player does not need help from the game to win). I don't think this makes smashville ban worthy (i mean i personally think only static stages should be played but it's not a big deal) but it's part of why smash brothers is such a janky game series overall when compared to other games that weren't designed by a guy who wants everyone to win. The smashville platform is literally designed to help the worse player, that's why it moves, because someone might get lucky and use it to their advantage. it's designed to shake up the game and make it a little weird.
Your point about customs is fair. I just think they are gimmicky and don't add any competitive depth to the game. The 'good' ones make a character a little better, which means a person has to try less hard to win, and only the worse player in any given matchup benefits from that. It's in the game, but since we have the ability to disallow them I find that to be better for competition. but, as you say, a good player will learn how to fight these new strategies so -shrug-.
the 2 stock vs 3 stock discussion I think could be very in depth. and I think it depends on personal view about the theory of what smash should be maybe. I like the 2 stock meta much more, it makes stocks actually matter, and it makes players want to play well so they can protect their precious stocks. players that are bad at staying alive or bad at taking stocks are punished much harder. it forces players to adapt very quickly (which i argue is a skill that great players have anyway), and I feel like in most 3 stock matches, you already can guess who's going to win halfway through the match and you're just waiting for the match to be over. I feel like more stocks just allows more opportunities for random weird stuff to happen. but I think this is very subjective I guess