Legends are a complex issue due to a lot of the reasons mentioned above, but they touch on another issue this game has which is a bit unrelated to gameplay - lack of communication from the development team.
Years ago, they teased a dungeon feature that would include legendary Pokemon and hidden abilities (which the competitive scene is in dire need of) but they have given us very little information or updates on this feature. A few years ago, Darkshade had mentioned he completed the first map for dungeons, and as far as I'm aware, that was the last we ever heard of the feature. They don't give us any ETAs on updates, or information on what they're working on. Since they first mentioned dungeons, they've added new regions, PvP features, and even a mobile version of the game. There is really no telling where they are on this feature, or if it's in their plans at all.
The last we heard from the team was in May, when Kyu mentioned they were working on more PvP-related things and they weren't sure what they doing beyond that. Not knowing what they're doing or if dungeons are still a priority makes it very difficult to create ideas for how to implement these features, so we're pretty much stuck waiting on them to do/say something.