Catchable Ultra Beasts mean they will be weaker, it's simple as that.
Pokémon games are too easy, that's a common complaint since ever. The legendaries are supposed to be these extremely powerful entities that you'll have to take down at the end of your journey, providing that final challenging BOSS FIGHT! Problem is: since legendaries are catchable, they can't be that strong. Being catchable means it can be used on PVP battles, so they can't be completely broken.
When the hard part of your big boss fights is not killing your enemy right away (so you can spend 20 minutes boringly throwing balls on it) instead of having to work your ass off to overpower it, well, that's underwhelming. Especially when the game keeps constantly hyping you about this creature having powers that could destroy the humanity/world, and when you meet it you can kill it with two hits using that incredibly common animal anyone can find outside their houses.
GF could easily circumvent that by making the legendaries stronger in the wild than they are after you catch them, but they never went that way, so…
Totem Pokémon go that way, but no one believes fighting a powered up Raticate will be actually challenging.
Then Ultra Beasts were presented and they could easily be the answer to that problem: if they are not catchable, then there is no reason to hold back on their power - they can be strong enough to require a whole team to take them down.
@GetsugaZoro:
The most I've seen are people saying the last evo should be female only because it's too feminine, those retards hating on it for it being too feminine are so few, that it had to be part of a Kotaku article.
I don't know which forums you are used to go to, but the ones I see are filled with people complaining about Popplio becoming too feminine. Not one or two guys, but a big portion of the players online. And that's not something that started with Brionne: ever since the final stages were leaked, people complain about Popplio becoming a mermaid because it would be too feminine and it would be ridiculous to play with a male Popplio looking like that.
It is also symptomatic of something bigger, so an article based on that is not only fine, but something I encourage. The Kotaku one is pretty bad, though, since it just mentions the fact and refrain from discussing it.