I've been playing this on and off since launch.
I'd say the biggest flaw is the amount of grinding you pretty much have to do to get anything done. And not far behind it is how expensive the pulls are for the gacha.
I spent a good amount of time saving gems so whenever a character I really wanted showed up, I'd hopefully be able to get them. I did after 80 pulls, but then I had to start upgrading them from level 1, which is just a slog.
Outside of that, it's good, especially for a F2P game. It just suffers from the same thing as, well, every game that tries to slowly bleed its users dry.