It's a decent game overall. It has, in my opinion, the best control in any Mario game , for fludd, combined with Mario's moveset, allows for great flow when moving. This great movement is best shown in speedruns; it is truly amazing how well Mario can get around (Which is why I'm so excited for Odyssey). The hub area also allows you to explore your movement options.
However, the game feels like it was rushed and could've used more time in the oven. Several missions are just plain frustrating to play, such as the red coin blooper surfing, watermelon festival, and Yoshi's Fruit Adventure. Sunshine also, ironically, has the worst water controls in any Mario game. In addition, the less I talk about the pachinko machine and toxic river the better. The game also lacks diversity in setting and bosses. Sunshine recycles the Blooper Boss and Shadow Mario chases way too much, and the tropical setting wore thin by the time I finished the game. I also found the game's plot laughable, for the piantas, after shadow mario appears on the statue, should completely free Mario (Since they can clearly see Shadow Mario). I'm not expecting a top tier story in my Mario game, but it should at least make sense. Of course, This criticism is all ignoring the unpardonable sin that is the blue coins.
Overall, I'd say Sunshine has great mechanics and underlying structure, but its execution leaves a lot to be desired.