I have to express this was an enjoyable experience up onto the point where the team has to fight underfell/evil Sans. It absolutely sucks you out of the experience by presenting you with an incredibly unfair fight that outright doesn't feel fun, no matter how many interesting mechanics are being introduced for the sake of a difficult fight, and it oft feels like it misses the point as to why the original Sans fight was difficult in the first place, only being put in here for the sake of doing a callback to a brutal fight that was meant to punish you for your own cruelty.
I love all the attention to detail, the way characters are written and crossovers are folded into the story, but evil Sans is the one large, ugly zit no one enjoys having to go through.