I got the same crashes. Mostly when pressing the synchronize chapters in a book. So I quit using the book feature and just made pages. I spent 2 days trying to redo it every time it crashed I had to save every 5 minutes and close the app to keep my changes...but I got fed up and just made it into pages instead of using the book feature. I'm also a new user of Affinity as well. A bit disappointed.
Aside from that, there is no full-screen preview like in InDesign (shift+w), and masters duplicate themselves every time I sync books (ending up with dozens of master pages).
I almost pushed my company to switch over to Affinity instead of Adobe because it is more cost-efficient...
Currently, my stand is a big no. It took me almost 2 days trying to go around fixes, crashes, and limitations that I could have done in 2 hours in InDesign.
Just hoping since it's just version 2, and could be unfair to complain to Adobe's apps at version 12 or something...but still a very unsatisfactory start for me.