Hello, I am a PTCG Online-only player and I'm glad XY12 Evolutions and its theme decks have finally been added in the game within the last 24 hours!
I tried playing both at Theme deck format Versus Mode at first and they are nostalgic AF. Particularly, having to rely on Sandshrew's Sand Attack or Electabuzz's until you get a good usable card next turn. Both theme decks are built heavy on Energy and equipped with barely more than 10 Trainers, just like way back in the day in 2000.
That said, it's saddening that XY12 Evolutions cards (at least in Theme deck format) aren't at par with the rest of the sets in the XY Era. While Beedrill, Hitmonchan, and Mewtwo can still compete, old staples Electabuzz and Magneton just aren't strong enough anymore. Electabuzz's Thunderpunch attack has weak base damage in today's standards, it could've been tweaked to retain its "rare" value from back in the day. Magneton's Selfdestruct attack isn't worth the recoil damage since another Theme deck from XY Breakthrough has the stronger, fully-evolved Magnezone. Even Raichu's Spark Bolt attack is outclassed by its counterparts from the Burning Spark deck or the online-only Crushing Current deck.
Perhaps Beedrill will do well in Standard format, with support from Forest of the Giant Plants, Ultra Ball, and Revitalizer. Mewtwo will probably fit in any deck, as its Psychic attack cost is quite versatile. Outside theme deck format, Raichu is unnecessary as Raichu-Break does better at an almost identical attack.
As for the other Pokemon, the Generations/Furious Fists Machop and Machoke are better IMO, though the new Machamp is a good attacker. Haunter would be an okay-ish attacker with Fates Collide Hypno and ChaosTower around, but 80 damage isn't that great. And Dugtrio from this set is quite slow. Its Earthquake attack is much stronger than its XY Base Set equivalent's, but generally it's too frail to last long in battle.
Last thoughts: I'm surprised they didn't add Brock's Grit and Misty's Determination in these theme decks, or reprinted some XY staple supporters. Also, 2 or 3 more Trainer cards on each deck would've been of great help. The way these decks were constructed was also too nostalgic, lol. And lastly, PTCG *HATES* the Bulbasaur line, and to a lesser extent, the Squirtle line.
It would've been great to see all the 3 original Kanto (non-EX) starters reprinted and modernized.