I have not tried any multiplayer or soccer in Mario Sports Superstars yet as of this writing nor have I really used amiibo cards, so this is more like general impressions of the game. I, however, have played nearly all past installments of the various sports subseries, including Golf's Toadstool Tour (GameCube) and Advance Tour (Game Boy Advance), Mario Super Sluggers (baseball on Wii) and Mario Sports Mix (Wii). So I have much experience with these kinds of games and can speak with an expert opinion. I also think the amiibo functionality is a rip-off.
The game's biggest selling point is also its biggest flaw. There are 5 sports, so there is variety. Unfortunately, none of them are really fleshed out. A good comparison to make for side-by-side value might be Wii Sports though there's more to MSS in terms of content.
Those looking for the variety in modes per sport will only get some generic challenges and limited stadium or course variety. Only 36 holes in golf from 4 different courses, plus 4 extremely similar and dull themes for 12 horse racing tracks, none of which feel at home in terms of the Mario Universe. No special items like shells or fire flower either. And the unlockable characters, OH GOD! You won't bring back Neil and Ella from Advance Tour or have some random inclusions like Petey Pirahna, King Boo, or anyone from WarioWare, Nintendo? But you will include Metal Mario and PINK GOLD PEACH!?!
So, why isn't this game awful and just average to me? Well, that's just it. It's not Super Mario standard, but the gameplay is solid. I especially like the pitching mechanics in baseball and taking care of horses in your stable. If anything, I think horse racing would make a great standalone title if they had interactions between Mario characters and the horse and expanded on everything else in the mode, including item variety, track themes, and customization. It just feels like Mario Sports Superstars should have been the start of a new franchise or even just a new subseries in the Mario universe with either forgotten or entirely brand new characters and just some Mario classics like Luigi, Daisy, Waluigi, etc. being unlockable bonuses. At least then, despite still being of average quality, it could than be looked at in an standalone context and then improved upon for an Nintendo Switch sequel. Instead, because of the Mario Sports stigma attached to it and lack of creativity to help back it, it ends up alongside more recent Mario and Sonic Olympic titles and general Nintendo spinoffs as a fun diversion for fans and newcomers. But one that will probably be forgotten and forsaken by said players who will probably replay older, much larger standout Mario sports titles instead.