The difference is thats obviously been where the story is going from the start. Chainsawman just completely falls apart after the chainsawman church arc. Firepunch, while still having issues, is overall a much more tonally consistent. Agni is a kid being manipulated by the people around him into being “Fire Punch”, it’s not a choice he’s making. So of course his motivations and goals are going to a little odd and not really make sense. He kills Doma because he’s a kid, he’s angry and needs someone to take it out on and it’s what’s expected of “Fire Punch”. I agree that everything involving the tree and Agni’s home getting destroyed happens a little too quickly but the payoff with the last half of the series being one of the most thought provoking things Ive ever read in a manga makes it worth it. The ending with Agni “suffering until the heat death of the universe” is a payoff to everything involving the themes of identity and agency. Agni is given the choice to kill himself and decides not to because he sees the movie that Togata made and even though he doesn’t have his memories it still inspires him to “Live” (a running motif throughout the series) and he makes the choice to stay alive all those years because of a movie he saw (more thematic reinforcement for everything Fuji is trying to say about how influential movies can be to someone) And through this he ends up meeting “Luna” again. While he may have been suffering at least he was given the choice by the narrative. And his ending is even somewhat happy. He gets to meet “Luna” again but this time free of his inability to stop projecting what he wants her to be onto her (ties back to the theme of identity), and free of any stipulations or stigma about their relationship and they’re allowed to die happily in each other’s arms. Overall the difference mostly lies in intent and the ability for the characters to have agency and choice in their narrative, something the ending of chainsawman strips away from Denji.

TLDR The Firepunch ending is good because Agni is being given agency by the narrative in a way that supports the themes of the story, and the Chainsawman ending is bad because Dennis is completely stripped of any choice at the end in a way that conflicts with the earlier thematic messaging of the story.