As best I can see, bookstack has not experienced much in the way of concrete issues with Github. And there there are no concrete benefits from migrating to Codeberg. It is his project, his perogative, completely. But the major disbenefit of being on some other forge is that people are less likely to find the software and so less likely to adopt it.

I use Bookstack for a family wiki. I probably would not have gone with it if it had not be hosted on Github as the visible activity on Github makes it clear that it's a project with momentum (18k stars, lot's of activity) etc.

I can't help but feel that moving will make the project less successful than otherwise...