Eh, Tennis is kinda meh.
Adventure mode is a pretty huge disappontment for me since there's basically no RPG elements involved besides leveling up, which grants you ever so slightly better mostly meaningless stats, Besides that, all the remaining customisation is getting more rackets which just increases your survivability against the super power smash whatever shots that ding your racket if you fuck up the timing.
Can't even choose your own character, it's Mario, you're Mario and you're gonna like it. Wildly fluctuating difficulty, but overall too easy. Crazy that endbosses are usually more on the easy side of things and usually lowly gimmick fights, and then you crash and burn against a random Koopa or whatever.
It's kinda completely forgettable since the "adventure" is just going where you're told and maybe once in a while optionally doing some challenges for better rackets. Could've done this with a simple menu, without the overwold map.
And some of the challenges really get long in the tooth. Example: Return 300 balls (after 10, you get a bonus, so it's actually less, but still, that's fucking dire).
Plus, sloooow vapid conversation "cutscenes" before and after every level, regardless of whether you have succeeded or failed. At least you can skip them after the first time by pressing +. Still super annoying since they add stress and punishment to failure and nothing at all to success.
Biggest gripe for me is the lack in control customisability. I'd love to be able to tweak the bumper and X button actions.
Online for me is all about getting owned miserably. There's kinda precious little to do. It works, sometimes, often, well randomly unless it lags you out because Switch Wifi is terribad - but after a while I kinda get tired of it either way. Especially since there's still a serious balance issue between the various characters. Or maybe I just suck. All I know is that I loathe playing against the ChainChomp. There's a bunch of characters that feel kinda overpowered, others conversely underpowered.
Also not sure about the super generous rescue lunge that lets you happily flip across the entire court at the (double)press of a button or tilt of a stick. Makes positioning kiiiiiinda pointless. Well not really... but it really drags out matches IMHO.