Thanks for the space to leave feedback, hopefully it's helpful.
First of all let me say the characters so far are pretty fun and varied. Power levels seem predictable although I haven't yet had a chance to try arenas.
My biggest complaint is the lack of resources. Everything costs the same currency so you have to carefully pick between summoning heroes, upgrading heroes, or upgrading runes. Since there are very limited options for obtaining the currency it's quite frustrating
The second biggest problem is energy. The cost of levels is way too high - or the rewards for leveling up need to restore all energy. Most games let you progress rather easily as a newbie. In this game however you run out of energy after about 5 battles. Since playing battles is the only way to get any of the currency required to advance, it feels VERY slow right from the start. Most other games don't start to feel like a grind until much later, while this game is a grind from the very start.
Overall, I'm hoping for much more content, better rewards, better pacing, and many fun new heroes!