RN used to have a lot of issues years ago (~2015-2016 or so) with different layout rendering between iOS and Android. Since they re-wrote their layout engine to be the same codebase across Android and iOS this has been 99% fixed in my experience. You might very occasionally run into a difference of some sort but it will most likely be minor. Of course, I would say if you've never done a RN project before and you plan to release both iOS and Android, please DO test your app frequently on both as you code along. If you complete 90% of the code on, say, iOS only and then try to run it on Android, chances are you'll run into more WTH problems than if you were checking along the way.