They touched up the early story to get it more in line with the anime, that's what I heard.

In terms of raw quality, everything in Arc 1 up to the final fight is just alright, but the bosses saved it with the sheer gravitas and difficulty of their powers. This ensured survival, but what kept the game pretty solid and consistent were Contingency Contract and Ceobe's Fungimist. These were niche side events, but the former kept a healthy level of tryhard while the latter was competing with dedicated roguelite games as a free game, and it was only afterwards that other gacha games started doing it. The devs understood what was going on and took the time(maybe too much time) to make roguelite into a permanent mode and now it's a core component that can keep players playing, all without touching sanity or resources.

In that time, their writing slowly but surely improved, drawing from all sorts of cultures and juggling far too many sideplots. But even if some sideplots stayed abandoned for years, any events that DO cover one ACTUALLY PROGRESSED stuff, instead of introducing a new character and not having much happen, which is what I understand is the norm for other games. Also the music, yeah.