Pay wall after pay wall. They don't give you enough mats to progress your characters. You need to level up your stars using gold, which also is used to level up your character levels and purchase the materials in the store that can't be farmed anywhere. Then you also need training mats which are in short supply (more $$) and now the events can't be completed without spending real $$. Pick a different game unless you plan to spend thousands (literally) to catch up