So pardon my ignorance but I thought React was already for web and React Native merely a set of tools to help port React web apps to native mobile apps. What is the difference between React for web and React Native for web?
React and React Native are separate tools, and they don't play with each other. You actually need to build two separate apps and you don't get much of any re-use. For example, you can't use Material UI for React in React Native, and you can't build a React component library and just pull it into React Native.
So if you want a fully cross platform React experience, you need React Native from the get-go and to compile it for web when you want web.
So anything you could do in React, you can also do in RNWeb, as it's a superset of React.
Of course, you'd need to use the RN components if you want to share code between web and native mobile. But there's nothing stopping you reaching a high degree of code re-use, and/or using React components for the web-only portion of a RNWeb project.