* Shards, the resource necessary to "Limit Break" units and gain the ability to unlock new jobs, are extremely difficult to get for higher rarity units. They're uncommon random loot in shops or randomly pulled via special pull tickets, otherwise you must pull a duplicate of the unit which gives 15 shards for that unit. Only the lower tier and story units have easily farmable shards.
* Battery hog. The game eats battery pretty quickly, though not as quickly as FEH/Mobius in my experience. It is a bit intensive though so your device may have issues.