Edit: Or right now since it's early you can out-skill other people while they're figuring it out and soon the people who spend more will have both skill and raw power and will dominate
Does it suck that you can pay to get stronger and make the game unfair? Yep, but I'm not convinced it matters if you're playing casually. None of the pay to win stuff changes the game play drastically. You're only really increasing the amount of damage you do, it's not like Path of Exile where some of the really rare items can enable a new type of build or trading card games where certain cards let you build different decks with different playstyles.
Personally, I see no incentive to buy anything. I can clear a level 12 challenge rift, grind slowly and slowly clear higher level ones. Or I could spend a bunch of money and get to do level 20 challenge rifts or whatever. At which point you hit the wall and either pay money or keep grinding. But since the game is all about grinding, why pay to skip forward and grind more?
I played a wee bit and it didn't seem that bad. You can pay to upgrade your gear, but that seems entirely pointless in an ARPG. The whole game is a grind to get better gear! Once you actually have perfect gear, why keep playing?
My issue with the game is that it lacks depth after having spent far too much time playing Path of Exile. There aren't that many possible builds, and the game basically tells you exactly how to build if you want to do PvP or PvE. As far as I can tell, those builds are the meta ones, so the designers are forcing the meta.
But as a game to run in and kill some monsters without thinking too much, it seems fine. Which makes sense since it's a mobile game.
[0] https://www.twitch.tv/wudijo