"Kirby is too easy!"
"Mario Tennis is too hard!"
Nintendo just can't win. Still the score is better than I thought considering the full roster and online isn't avaible.
To be fair, the complaints go beyond difficulty spikes. The adventure mode is short and some found it repetitive, its RPG elements are very superficial (leveling etc), offline tournament mode is extremely limited (so single-player options in general are limited), local multiplayer is limited in terms of options & level-selection, local multiplayer utilises split-screen (which seems unnecessary in a tennis game), and, judging by the pre-tournament demo, the online performance is a bit sketchy - though to be fair a number of reviewers are with-holding final judgement until a day one patch comes through.
It sounds like the core mechanics are very strong and that the game is, broadly, a return to form for Camelot - but it also sounds like there are strange limitations and that, as a single-player experience, it's still shallow despite the addition of Adventure mode, so it's probably more akin to Arms and Splatoon (though Adventure mode can be done in 2-3 hours apparently) in that it's best enjoyed as a multiplayer title. I'm still getting but am a bit disappointed by the limited single-player content and confused by the possible limitations with local multiplayer, but these things could easily be changed if there's post-launch support beyond extra characters (it won't take much to add extra settings and options to offline tournaments and local multiplayer matches).