Any deck, no matter how overpowered it may seem, is only as good as the user is familiar with it.
I would fail using Darkrai/Hydreigon (worse than I already have, mind) if I hadn't tested it as extensively as I have.
PlayTCG is a way to do that whenever is convenient, without owning the cards, with people of all skill levels from across the world. Why isn't that awesome?