The role of the Ultra beasts in the upcoming meta should be very interesting. While some of them like Pherosma, Kartana, and Zerkitree seemed broken on first glance, After taking a closer look, they each have issues that prevent them from just plowing through teams. Pheramosa isn't as extreme as Deoxys, and has no moves as spamable as psycho boost. Kartana has awful special defense, exploitable weaknesses, and all its moves are only middling in base power. Zerkitree has power, but everything else is pretty middling. I think the main thing that could make or break these pokemon is how effectively they can utilize beast boost.
Zygarde 50% with power construct seems fun, as it basically gives Zygarde a super sitrus berry as an ability. Should be a great set-up sweeper.