There are other comments in here comparing react-native to Electron. Do you know if you could build Nylas Mail at the same pace with react-native? Will the binary size/RAM usage drop significantly on react-native?
I haven't yet seen a substantial desktop app built with React Native and afaik neither FB nor GitHub is investing in React Native for desktop so generally this is hard to say. React Native is more of a framework whereas Electron is a runtime-- much different goals though both are super cool and I'm enthusiastic about the future of both!