* Cryptic error messages. Out of maybe hundreds of errors/crashes I had while developing a medium sized app, only a few of them provided useful info about what/where happened.
* Lot's of outdated/abandonware/low-quality libs in the ecosystem. You really have to be careful when picking the dependencies.
* Fragile and complex builds. You'll have to use dependencies to make anything more than a basic app (unless you want to DIY), so get ready for some "fun" when upgrading the project because not all of the dependencies will be upgraded at the same time, might also often have the APK/IPA or development builds fail because of dependencies not properly installing or not playing nicely with one another.
* Lots of little stuff. For example, I had the ios simulator logs not work out of the box when launching with "react-native log-ios", so had to use a third-party solution for that. Metro bundler sometimes just breaks. I still have a problem of the app randomly closing down after some time on iOS, which I did not look deeply into yet but will have to solve before release. Constant issues with nesting scroll views on android. Hard to predict cross-platform inconsistencies in layout behaviour in some places. Can't name everything right now but they played on the nerves throughout the development.
I hope it gets better some day, but until then I'll probably look for alternatives for my next project as I wasn't very happy with RN.