Balance issues...
Arceus and Dialga decks are too strong. There is very little counter play to defeat them as they are massive tanks and Dialga has the ability to give energy to your benched pokemon. There is very little incentive to play any other deck and the only one that could possibly beat them is another big EX deck which is a grass deck consisting of ex Celebi and ex Exeggutor (I also find this deck problematic). This hypothetical deck though has more random coin flips so the consistency is less.
Palkia is a other problem, but I don't see it as rampant as Arceus and Dialga although probably has a high win rate.
What is the win rate when it comes to a certain combination of cards in a deck? If I had to guess if the deck contains ex Arceus and ex Dialga it is winning easily above 60% of the time.
Unsure what is in store for this game, if there are sets rotating out or seasons where certain cards are banned or not allowed in the monthly event that could fix the problem.
If the game is going to have just an open pool of cards for anyone to play with the power creep needs to be insane next set or it's going to be the same two deck with minor changes and they will keep racking up a huge win rate.
Personally what is the point in playing this game when I am going to get steamed rolled by the same combination of cards no matter what I play? I see it as play the only viable meta deck or play a counter deck that it's only ability to win is high roll to oblivion, be insanely lucky (I am not), or just accept I am going to have a 20-30% win rate.