At a guess, I'd say "Side effects in render functions". People might be doing things in render that aren't generally considered safe or wise, and forcing concu

At a guess, I'd say "Side effects in render functions". People might be doing things in render that aren't generally considered safe or wise, and forcing concurrent rendering on them would run that code multiple times immediately, instead of the expected "once per render". The new version apparently doesn't do that, unless you specifically use one of the new features, which means that you have accepted the other caveats for that scenario.