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Even though they are for all intents and purposes legendaries, the UBs have a different kind of feel to them, in my opinion. They feel more like Snorelax in Gen I: Strong Pokémon you can only catch a limited number of.
There are a des reasons why I didn't like huge numbers of legendaries in a game is that in past games, but the UBs don't have the same problems. It was very tedious to get usable legendaries in the past, but now they come with three perfect IVs and you can use bottle caps on them. The 700+ base stats ones just seemed to trivialize the game and are not interesting in competitive play, since most of them just win by overpowering things, but UBs all have pretty exploitable weaknesses. Finally, I disliked the idea of singular Pokémon. Most of the time, only having one of a Pokémon in existence seemed unjustified, and seeing them in comptitive games over and over was kind of jarring and unflavorful. UBs avoid this problem by not being implied to be unique creatures.