The thing with Diance is that if you want to have it as a consistent opening you'd want to run more as you do Ralts. If you do that it also needs to have a significant purpose in the lategame (like Ralts to Gardevoir GX has, or Sylveon GX for that matter).
Sylveon GX can be disrupted by N, at the same time it can also find N to disrupt your opponent. As such there is no significant disadvantage there, your opponent needs to have N or fall under the force of the 3 cards you've searched for.
One of the reasons why I personally dont use Wally and wont consider him is that like Diance there is a set time to the card being good. One of the key approaches for great decks to me is be filled with cards that are both relevant early game and lategame. Gardevoir GX fits that bill, so does Sylveon GX and frankly speaking there are few things as backbreaking is playing a Guzma and attack with Plea GX.
Sylveon GX can be disrupted by N, at the same time it can also find N to disrupt your opponent. As such there is no significant disadvantage there, your opponent needs to have N or fall under the force of the 3 cards you've searched for.
One of the reasons why I personally dont use Wally and wont consider him is that like Diance there is a set time to the card being good. One of the key approaches for great decks to me is be filled with cards that are both relevant early game and lategame. Gardevoir GX fits that bill, so does Sylveon GX and frankly speaking there are few things as backbreaking is playing a Guzma and attack with Plea GX.