Just did an upgrade, and everything seems to be fine. But I recommend to do what they wrote in the instruction - disable all third party extensions before upgrade, even if using only FoF extensions. I had problem, that just after the upgrade I get an error:
Fatal error: Uncaught Flarum\Http\Exception\RouteNotFoundException
while trying to access main page. After I disabled and enabled all 3rd party extensions, problem had goneπ