Seen a lot of people suggesting there may not be a lot of content there for single-player gamers.
For what it's worth I've played it and there's a lot there to keep me busy.
Grand Prix mode: every fighter gets a trophy next to them when you win it, and each difficulty has its own (stackable) trophy too, meaning to 100% that you need to win Grand Prix on Level 7 with all 10 fighters.
Given that starting from scratch (with no experience) I beat a Lv 4 Grand Prix in about 45 mins I'm estimating around 10hrs to pull this off.
You've then got the matter of unlocking every arm for every character. There are 30 unlocked at first (three for each character) and 300 in total (you unlock every other character's arms, so 30 arms x 10 characters). Completing Grand Prix mode gives you enough coins to unlock 5 or 6 of these each time.
Finally, 1-on-100 mode, which is deceivingly difficult, is separate for each character, and each character needs to unlock every stage by scoring 100 on each.
So that's:
10 x Level 7 Grand Prix trophies
300 arms to unlock
100 x 1-on-100 matches to complete (10 for each character)
Dunno what other reviews are saying but that's going to keep me busy for a hell of a long time without having to touch online, and that's before any new characters are added.
My review:
https://tiredoldhack.com/2017/06/07/arms-switch-review/