I've actually been learning a lot playing the adventure mode. As someone who sucks at Tennis games, the levels forced to me understand how important it is to control ball placement, how useful retuning to the center after returning the ball might be, how to get the timing right to block zone shots, etc. It doesn't do a great job of explaining these things to you, but without getting better at these I wasn't able to pass the levels.
Not being able to retry is annoying, but I'm pretty sure it's there so you don't just grind and grind. The beginning conversation of a level is skippable and the one where you lose isn't for a reason: when you lose, Toad usually gives you hints about what you're supposed to be learning/getting better at to finish that level. They could let you retry before finishing without winning xp and Toad could give you the hint without exiting to the map, next to a retry option, that would've been a better solution.
Also, the bowser nets return the ball with a strong "swing" that pushes you back if you don't respond with the correct swing. The ones with stars let you do zone shots, which obviously let you aim, but also destroy several nets at once. This is pretty obvious from trying it once.
Timed bosses make sense because you lose time for making mistakes, in other words, the idea is that you are able to reach and return the balls correctly. Where the punishment in a regular game is the opponent scoring, here you lose time. Seems pretty logical to me. Not to mention that the adventure mode is also supposed to put people in different situations than a regular tennis match to switch (pun not intended) up.
As far as online goes, the lack of more options for game duration is absurd but I was actually surprised at how the game lets you easily create rooms and set or search for the rules you want. It seems pretty sturdy and functional, more than usual in a Nintendo game, you can even see your opponents connection status before accepting the match, something missing from most or all of Nintendo's online games. Low bar, I know.
Other than the duration of the match (which is a pretty big omission), what exactly are the options missing? Because I see people saying the match options are barebones and I see plenty of options, so it kinda seems to me like people are focusing on one missing thing and ignoring everything else. Maybe I'm missing something.
I haven't finished the game yet, still need to play some more, I agree that the adventure mode is very basic and barebones and that there are a few things missing, but this thread seems disproportionally negative to the game I'm playing, which has really strong core mechanics, plays good, looks good and seems to have a reasonable variety of content.