During one of the many debates about Switch online and the lack of dedicated servers, was like a year ago so hell if I remember any specifics. I see how it could make sense though, when playing a 1v1 game like For Glory, dedicated servers would be a extra connection point the signal has to pass through. It's mainly worth it with more players as there's bound to be atleast one with shaky internet. I could be completely wrong though, and even if not, dedicated servers would still be good for 3-8 player smash.
I think it depends on the game. Something like Smash, I don't see why P2P wouldn't be fine most of the time. It's not like we're talking World of Warcraft with scores of players interacting at once. It probably also means less chance of the publisher shutting down online functionality after a few years. Though, even with P2P, we're still talking about connecting through a company server, so that's not a guarantee.