Miltank looks like a very good card, but I think it also comes with some playability concerns. I'll be focusing on a Gardevoir GX deck as it seems to be the obvious deck to play it in.
The first is bench space. With 2-3 Gardevoir GX, usually at least 1 Lele to search for a supporter at some point, maybe Octillery for draw power, and Diancie/Sylveon/Alolan Vulpix for your set up option, you're not left with a lot of room on your bench. Worst case scenario, I guess you could ditch the Octillery from the deck list, but whether or not that's worth it comes down to playtesting.
The other concern is over extension. The best time to play Miltank would be when you're still setting up and you have fresh Gardevoir GX on the bench with no energy. But once you get a Gardevoir GX to 4, 5, 6 energy to start swinging for OHKOs, every energy you add on beyond that point for the 90 recover becomes over extension. This is especially dangerous in the mirror match since your opponent can just Guzma your over charged Gardy GX into their 1 or 2 energy Gardy GX, and there's an OHKO you can't recover from and all of the energies lost to discard, forcing you to use the GX attack to get your resources back.
I think the main question to be asked with Miltank to clarify any of these concerns is "how many heals do I need to impact the game enough in my favor?" I believe the answer to that question would determine its usefulness.
I believe Miltank's role is to allow you to set up early game without having to worry about your Gardevoir GX entering the active with only half HP remaining. It's very good at protecting you from Alolan Ninetales GX sniping your benched Gardevoir GX early on with Ice Blade. However, it naturally has limited use because you do not want to be attaching almost all of your energy to a single Gardevoir GX and making the rest of them worthless. If Guzma wasn't a thing, I think that would be totally okay, but since Guzma is a thing, you really do not want to be putting all your eggs in one basket.
Hopefully without taking this into too much of a tanget, I prefer Acerola as a restoration option over Miltank, at least in concept. She can scoop up your damaged Gardy GX, all the evolutions below it, the float stone/choice band, and all the energy as well giving you a refresh and a huge handful of energy that you can Secret Spring onto the rest of your Gardy GX. I personally feel that because Acerola restores not only your Gardy GX but also your resources is what makes her a superior recovery option, that is assuming using Acerola is the best usage of supporters that turn and you have a way to make her available when you need her (Lele or Skyla/Sylveon if you don't get N'd).
All that being said, I would really like to try this card when it comes out.
