The good thing about Complete Zygarde's transformation method is that once it does transform into Complete Zygarde, it regains a sizeable amount of its HP back, which means post-transformation, it'll be pretty healthy (and its stats go up hardcore, as stated above). Not only that, but Zygarde is actually fairly bulky which means it can probably take a hit that can get it to less than half HP, and survive (at least the 50-percent form is, not sure about the 10-percent form). This is unlike Darmanitan who has a really bad defensive typing and is also incredibly frail, which means it dies to pretty much any non-resistant hit, while Zygarde has the sheer bulk to survive a hit that isn't a super effective attack.
I'm honestly kind of disappointed with the transformation method they used for Zygarde, but at the same time, the HP recovery the ability grants+the stat boosts make it passable, and Complete Zygarde is still a perfectly usable Pokemon on its own post-transformation (if not possibly insane, as I said in a previous post, Complete Zygarde post-transformation is probably gonna be absurd and potentially Uber/AG material). I would've preferred if Complete Zygarde was a complete standalone from 50-Zygarde and 10-Zygarde, but this method is passable since it heals a sizeable amount of HP back allowing Zygarde to come back as a completely new Pokemon in Complete Zygarde, and it increases all of its stats too. Zygarde also has the sheer bulk to make use of its transformation ability well, as it's actually very bulky, which is something a lot of people don't seem to notice.
So yeah, it's not a complete waste, but Game Freak did kind of make it a silly transformation method and as always, some stuff they do is always inexplicable. But...the way Zygarde is built+the additional bonus of HP recovery from Power Construct make this at least passable, and Complete Zygarde could be completely usable both in-game and competitively (and could end up as Uber/AG material in competitive).