I think another point to be noted, as far as being able to devolve your opponents Pokemon within a turn and still act, is when we consider evolved Pokemon with static Abilities that harm our potential to interact with the opponent.
For instance, you can evolve a Porygon2 and bounce Vileplume to the opponent's hand, regaining the ability to use items. Or Trevenant XY for the same effect.
Within the scope of the Sun and Moon block, where I am convinced this and other cards are being designed and balanced around, it would allow you to devolve Power of Alchemy Alolan Muk, so that you could use the ability of a Basic Pokemon. Maybe bounce Alolan Dugtrio or Toxapex so that you can retreat safely.
Any ability that reduces damage taken can be undermined as well, like Safeguard Alolan Ninetales, Armor Golisopod, Daunting Pose Machoke, but in many ways those are situations where devolving itself will guarantee the KO (Machoke the more likely to be benched)
Then there is of course the fact that, as we move forward into a world with potentially no Item lock and more emphasis on Stage 2 Pokemon, Rare Candy will be a more important card, which means Porygon-Z can punish heavily by devolving a Solgaleo GX into Cosmog, not Cosmoem. Not only does that bring it much closer to KO range, but it also makes it all the harder to reestablish.
All of this said, do I think Porygon-Z will have instant effect into the game? Probably not at all, the way decks are built would have to drastically change for it to be effective in PRC-BRS, but I do think that it serves an interesting function as years go by. I could see it becoming something interesting to watch out for in the coming two years.
And last bit, we really need to see the whole evolution line to really value this card. If Porygon or Porygon2 have any strong use cases, it would push the value of Porygon-Z by a lot. Only 2 of 8 (9 if we count Dark Porygon2) Porygon2 have been printed without a Pokemon Power, Poke-Power, or Poke-Body, so chances are rather high that Porygon2 might have something cool to it. There isn't a particularly good pattern, except that most have been bench sitters with repeatable abilities (Change and energy's type, draw until 3/6, return a tool to hand, use a Supporter effect)
So let's hold out for a really strong Porygon2!