The game's 2 and 3 rarities drop rate is too low to which, you dont get enough fragments (purple gems) to buy character specific fragments to promote your own characters.
Sure it's not impossible to fully promote a character in casual play. But the game throws a whole bunch of types of enemies at you with rock, paper and scissors mechanics while having you manage environmental bonuses and school bonuses. Even if ignoring environmental and school bonuses, your resources are spread thin within the few characters you have.
Oh and I hate a curve ball in one of the early stages where your effective team is suddenly ineffective against one group of enemies which kills you pretty fast or can't kill them in the allotted time which prevents you from getting your 2nd map star.
But despite all of this, I still log in daily. I am playing this game without reading guides or following metas. I'm sure I'd have better results if I were to follow someone's guide but, I won't be able to enjoy it otherwise. Also, I don't read Japanese so the only thing I can do is screenshot and then dump it into the Google Translate app.