You know, I have a shrodinger opinion about UBs!? I both like and dislike the concept, and I dont know when I will settle for one xD.
On a serious note.
I like that:
-We may not be able to catch them.
-They are probably very strong.
-The idea is quite creative.
-Nobody "knows much" about them (I guess there are probably a few NPC that know something about them).
I dislike that:
-They may not be catchable.
-Even though the idea seems creative, we still have yet to see how well it is implemented. And I have been disappointed more than once.
-Now we have to deal with the question: "Is it or is it not a pokemon?"; I always imagined humans were pokemon in the poke-universe, but with this it seems clear that is not the case. And that gives us another question: Where do pokemons come from?
For the un/catchable part, I would prefer if they are catchable, but only on certain circunstances. The same applies to legendaries. For example, you have to defeat it at least once, and after that it will be a very hardcore encounter on a certain location, but you can only catch it once(?) I dont know actually, something like that, I dont want to be forced to catch lunala or solgaleo on the first battle (on the other hand, being incapable of catching them in the story, but after the story...), but still being able to catch them after a while (for example, ORAS deoxys, kyogre and groudon, which you can battle again after you defeat them). It would also be awesome if they are on a high level compared to your mons (say, you battle them at level 80, but your mons are at level 50 top).