Where Retcons are good and where they are bad

Retcons: a tool used for tying stories together, dealing with changes, yada yada yada.

The question is, when are they a good thing, horrible, or either or

For example, people generally think that retconning Thor and Apocalpyse into having fought once is a good thing due to the fact that they are both older characters

Then we have a retcon that can go either way, like with Ahsoka Tano. True, its a bit harder to swallow at first because of how it changed decades of canon, but over time the writers gave her more time, made her work. When a character originally widely disliked elates a 90% tear jerker response at her leaving, you know that people actually care (And the other 10 %...well Star Wars fanon is about as divided as their galaxy). The retcon worked, helping mature Anakin from Episode 2 to Episode 3 and giving him another reason to have the issues he did that caused Darth Vader to be born

Now, for Retcons people utterly despise.....

Oh yes. Derailing characters for no reason, squickyness, retcons that the next writer does their best to retcon away while the fans try to ignore completely

True, some of you may like Norman-Gwen shipping, think that Apocalypse and Thor meeting sucks or something. But here's a thought, what makes a retcon good, what makes it bad, and when should they be used?