The pacing of it is really off. Specifically with the final group, difficulty jumps up and down arbitrarily. Some bosses' mechanics are so radical that they make fights huge chores, then their immediate successor is just a standard kill 'em to death fight. It's off-putting. For example, the 2nd to last fight is by far the hardest, meanwhile the final fight is standard in every way.
I'm overall content with Grim of Abyss as it feels like a contained experience like its sister modes, but with odd balancing on the bosses and absolutely no payoff for completing (unless something changes upon beating the 32 bosses in a row, rather than one to three at a time), it's kinda disappointing.