Netmarble keeps releasing new units every month or so, resulting in recent and old units quickly becoming irrelevant. the bird and the deer boss which are not fun, and requires specific units to beat. they updated the training grotto, though apart from adding a skip feature once you clear some missions, it is basically the same. activities are boring and offer little reward. They need to start adding new fun rewarding content and revamp all the older units to at least make them usable.