The biggest sticking point being the card rarity based progression. It is a completely random "pay to get" model gacha system. This is because the rarity progression system is based standard diablo style rarity progression and color coding.
Standard/Common -> White/Grey
Uncommon -> Green
Rare -> Blue
Epic -> Purple
Legendary -> Orange
Mythic -> Gold
The only way to obtain a better rarity is through loot boxes (aka pinata Llamas in game). While you can make a Common hero card better through leveling and evolving it, even a maxed out common card is going to be absolute crap in comparison to a untouched Mythic rarity card.
The only way to obtain higher rarity cards is through loot boxes. Loot boxes are rewarded through game play, about 1 every 2-3 for hardcore players or about 1 a week for casuals (once ou get beyond the initial easy to do content that rewards them more often and are into the grindier endgame portion of the game). Still the randomness of the loot boxes means that you could potentially go years and never see a mythic drop. Or you can get 5 in a row! It's really that random. Whats worse is everything in the game is a "card" to use. The hero you play, the weapons you use, and the traps you create are all cards with rarity + level progression system. Since there exists no system outside loot boxes to obtain better rarity cards, the game feels like a horrible cash grab.