So anyway, after a few hours of playing Under Defeat on Switch, it's safe to say this is a pretty solid port, at least in terms of running well. No lag that I noticed (so maybe 3-4 frames?), all modes and helicos unlocked right out of the gate, and I'm still laughing (but in a "but also, this fucks" way [tvtropes.org]) over the SuperSweep soundtrack that sounds like it would go in a racing game.
(I heard there are discrepancies vs the Naomi version but tbh that isn't going to matter much if you're not trying to chase community high score tables or world records, and also slight inaccuracies are an inevitable part of every video game port)