Hello! Welcome to the forum!
Simply put, they are different games:
Scarlet and Violet are the current "real" games, which means they get the events and they are meant to last longer and they have all the new Pokémon. Probably next year a DLC for this game will be released, making the game last longer (but obviously more expensive).
On the other hand, Pokémon Legends: Arceus belongs to an older generation of games (so no new pokemon and less pokemon in general).
Other than that, Pokémon Legends Arceus is just a totally different game idea compared to older games: it's meant to be played after Pokémon Diamond/Pearl/Platinum or after their remakes: Brilliant Diamond and Shining Pearl.
It's not necessary obviously, but it surely it gives more importance and enjoyability to the game.
It has to be said that while Pokémon Legends Arceus is an Open-map game (only some areas are completely free to explore), Pokémon Scarlet and Violet are totally Open-world, which means every area can be explored freely. Now, this is subjective, but in general Open-world games tend to be more fun. I haven't finished pokemon Violet yet, so I can't say which one is better (for now I prefer the exploration in Pokémon legends Arceus), but the not outstanding reviews for the new games are mainly because Pokémon Scarlet and Violet have some technical issues: low framerate, some bugs and lags. Now, this is subjective as well, but while it's true that those issues can be annoying, they don't ruin the fun part of the game.
Many players are angry at the new games especially because they are underperforming compared to some other Switch games and while I recognize this is indeed an issue for the game, it's also true that it won't compromise the fun part of the game.
Pokémon Scarlet and Violet, with their open-world structure, are indeed innovative and interesting games to play.
I think an adult could probably enjoy both Scarlet/Violet and Pokémon legends Arceus considering how different from the older games they are.
I hope this will help!