Non web-devs seeing the word "JavaScript" and running away. Or stated otherwise, having to learn React + the JS ecosystem (minimized within RN) on top of all the mobile specific stuff.
Pretty good point, haven't thought that way since I was a Web dev, then React dev, so transition to RN/W was easy.
I had no previous experience with Android or iOS or the languages and was able to get caught up, but there was a curve to figuring out code signing, the app stores, deploying, permissions, etc.
For those non web-devs, they will have to learn some of web. You will need to learn the tooling like the other platforms. You don't really need CSS knowledge or HTML just basics. RNW implements RN stylesheets which is a streamlined subset of CSS.
The main thing is learning React, but SwiftUI was inspired by React so it should be easy for at least iOS devs to grasp it.
Non web-devs seeing the word "JavaScript" and running away. Or stated otherwise, having to learn React + the JS ecosystem (minimized within RN) on top of all the mobile specific stuff.