Look at the new Discord app. It's significantly slower than previously ever since the switch to react native, and has significantly more hangups.
React native works maybe fine if your target device is the highest end iPhone, but if your users are on Android One devices, it's not going to work nearly as well.
That's aside from the layouting being obviously off compared to the native layout engines.
I'd love to, sadly it doesn't appear to be open source ;)
I can say from comparing pre-RN and post-RN apps side-by-side on a Nokia 6.2 it's definitely much slower (scrolling immediately drops frames, opening/closing the keyboard and other reflows e.g. multiwindow cause the app to hang for seconds).
React native works maybe fine if your target device is the highest end iPhone, but if your users are on Android One devices, it's not going to work nearly as well.
That's aside from the layouting being obviously off compared to the native layout engines.