How serious is this game at a competitive level?

Recently started playing this game and I'm having a blast to be honest. Bought both expansions and worked my way through them on a single afternoon. While playing them I couldn't help but notice how most of the "hard levels" weren't actually hard but about RNG. Whenever a level looked very complicated I just looked for a guide and made an "ideal" deck to beat it, then again I had to retry several times until I got the cards I needed.

So, how does this game fare at a competitive level? You don't know what cards you are gonna get, you don't know what your opponent has (you can assume what he has but still you may get surprised) and evne if you did you don't know what's his hand or what's he's gonna get in the future. Can RNG straight lose/win you a game? If yes, how does Blizzard manage this at pro level? Best of 15?

Thanks in advance, forgive my ignorance.

Edit: For the record, not talking about money but about the game itself.