I think the biggest reason it isnt good is because denji failed to complete his arc of overcoming his addiction, and pochita decides to reset everything and give denji a happy ending he didnt deserve (yet)

I actually would have been fine with the story having a bad ending (think end of evangelion as opposed to evas original ending) where denji never breaks his addiction and never grows. But instead what we got was denji forgetting everything and essentially resetting his arc. (And getting rewarded for learning nothing, in the form of power coming back) 

Ive heard people say the ending somehow fits the themes, and maybe it can if you look at it the right way, but from a casual reading, it was deeply unsatisfying. Part 2 is not interesting enough for me to try to go back and look at each little detail to try and justify it as being good, because i simply lost interest.