If I get this game, it will be on Switch as that is where I get most of my 3rd party games that come to it due to portability.
As for the Online, I am sure it will be fine. I have no problem finding matches in games like ARMs, Dragonball Xenoverse 2, and Dragonball FighterZ so I am sure this will be no different.
Plus with Smash breaking sales records and moving a ton of Switches, the player base is only getting bigger and therefore more and more people will be playing online as well. I mean it is a fighting game driving sales right now, so one would think that many would be looking for more fighting games to play as time goes on.