So much hate I see here.
Still not Turn 1 friendly
Why does it matter if you cannot play this thing turn 1? As others are stating, this seems more like an endgame trump card (or cards, lol). And as others are saying, it'll probably need some sort of energy acceleration to work.
I personally don't think they're bad. I think it was you who talked about Legend Box, but as far as I know, back in the day that card was a meme at most. At least you don't need them in your hand all at once, you can play them from the discard. And in that sense, even if you prize a part, I don't think is that terrible as long as the rest of the deck can function by itself with 56 cards, which I don't think is that difficult to do.
Because, are they worh it? I'll write my own analysis. As I always say, I'm by no means an expert:
Greninja V-Union
Union Gain: It has 300 HP, so taking into account V-Union will probably be "endgame trump cards", it might not be that bad to use this move once so you can do something meaningful, but having an energy acceleration engine prepared is probably far better.
Probing Foot: I've been wanting something to reliably look at your opponent's hand for so long! I remember playing with Fossil's Omanyte when I was a kid, hehe
How many times does it happen that you Reset Stamp (or Marnie, since this is rotating) your opponent's hand so they have less chances to have that game-winning Boss's Orders, only to give it to them? If V-Union really are "endame trump cards", having this ability in combination with Marnie can be great!
Aqua Edge: 130 for just one energy. That's far better than 150 for 1 in RS Urshifu VMax, without having to make retreating shenanigans. Althought, for an "endgame trump card" it doesn't seem like much, at least you can attack for one energy, lol. It might allow you to finish of your opponent's second VMax with just one attachment.
Antidote Jutsu: Well, it is an extra, better than nothing!
Twister Shuriken: Okay, so, how can someone look at this attack and say it is bad? 100 to EVERYTHING!! They really want to end small one-prize support pokémon's existence, for real. And again with the "endgame trump card" theme: this is AWESOME to finish up games people.
Shinobi Body: Well, so you're not affected by hammers, nor Pokémon Catcher... I don't recall if there's anything else meaningful here. Being immune to those two might be nice though.
Waterfall Bind: 2HKOs any VMax all while avoiding manual retreat. I think it's not bad!
All in all, I think Greninja V-Union is a card designed to play towards the end of the game, that can really put its weight to close it. It might benefit more from Frosmoth than Melony as an energy acceleration engine, so it can use its three-cost attacks right off the bat. And since it's played from the discard directly to your bench, that makes it easier for Frosmoth to do its job.
My first idea here is maybe a Rapid Strike Inteleon VMax deck with Baby RS/SHH Inteleon on bench. You slowly chip away at your opponent while using Cheryl to heal, and you finish off the game with a big 100-to all bench attack, or a 180 attack to the opponent's active.
Zacian V-Union
Union Gain: Same thing.
Wise King's Dance: Ok, so this move is just INSANE people. It makes Zacian V-Union literally unKOable outside of weakness. And you laugh at RS pokemon's faces. Oh, and do you remember Ruster Sword? Oh, of course you do. Just hope it doesn't work with this monster, but I think it DOES. Not that it is easy to use on it, but...
Steel Slash: And if you just want to 2HKO things, you've got that as well!
Master Blade: OHKO anything.
Just. broken.
Metal Saucer and Bronzong makes this monster absolutely playable. I'm now thinking about a possible Duraludon VMax/Bronzong deck with Cheryl that slows down your opponent until you can finally get that OHKO machine into play. Then it's the end of the game for your opponent! Unless they are playing fire-types, and even so, the first move and/or coating metal energy con cover you from them as well.
Mewtwo V-Union
Union Gain: Same thing, same.
Super Recovery: Healing 200 damage is more than enough to nullify almost any attack in the game. But your opponent can still gust your benched pokémon to KO, and if those are really "endgame trump cards", I don't think that move will be too useful. Zacian's Wise King's Dance is far, far better.
Psysplosion: Well, that can also be useful in finishing up games, but probably not as much as Greninja's attack.
Photon Barrier: It might be useful in certain match-ups...
Final Burn: It doesn't even OHKO anything, since Tag Teams are rotating.
I think this is the worst V-Union out of the three. It can benefit from Shadow Rider Calyrex VMax, but... I'm not sure it's really worth it.
Now, if those V-Union come in special decks, we won't need to worry about having to pull them from packs. It is like this in Japan, so I hope we get the same treatment here.
Aaand again a long post. Thank you to anyone reading