Not everything written for mobile in React-Native would work in web mode. It's not just because React Native Web is newer, but also because Web APIs are not equivalent to iOS/Android APIs - some features are simply not there for usage. Here is the compatibility list of React Native Web, in the official website.

The lack of supports for both Web and mobile together also needs to be considered here. Take react-native-maps for example: it's written on top of Google Map SDKs for Android and iOS, and MapKit in iOS. It's entirely dependent on the mobile ecosystem, so if you want your map feature to work in Web, you have to implement separate solution for Web only.

Overall, you need to know for each of your project requirements whether the related feature/library is supported in both web and mobile, and if there is not, do you have enough time and resource to implement the missing piece. If the answer is no, then you should separate your code base to handle web and mobile instead.

My personal opinions...

What approach do you recommend for new projects?

I think new projects should use RSD as much as possible. It increases the incentives for RN feature development aligned with web APIs, and ensures that the web experience is as good as you can get with React. There is no investment at Meta in RNfWeb by either the Web or RN teams, whereas both have been working on RSD.

Will [RNfWeb] development be continued?

I will continue to review PRs and merge fixes. But I don't expect to put significant time into major development initiatives.

I'm unsure of how far from production ready the native part is...

I think it's production ready. The native part is driven by existing RN APIs, and the newer web APIs in RN are already used in production by Meta.

I think React Native for Web should be based on RSD...

That would require a lot of work and changes only for RNfWeb to still have to layer on all the transforms needed to match RN's non-standard APIs. I can't really see the benefit given the cost involved and the issues it would create for existing RNfWeb users. RSD on Web is just RDOM + StyleX.

Edit: I also want to add (given something that was said at React Conf 2024) that using RNfWeb to bring existing RN apps to Web is not a good use case. We tried this internally at Meta and found that RN apps are simply missing too much semantic information to produce quality web apps as output.

RNfWeb has always had the expectation that it be used for web-first development, where the extra semantics it provides for web simply no-op on native. This is exactly the same development paradigm that Microsoft have for RNfWindows - you can't simply run an existing RN mobile app on Windows, you need to use the RNfWindows API extensions otherwise you get no mouse support, no multi-window support, etc. This is one of the fundamental problems with React Native's JS API today - it's not a cross-platform library. And that's the reason why I started React Strict DOM; the API RSD provides has the built-in semantics needed for web apps, windows apps, etc. This makes it far easier to adopt as a general cross-platform API - you don't have to first pick the right "flavor" of React Native for the primary platform you're targeting.

So the idea that you can pick either RSD or RN JS to author cross-platform apps is incorrect. This was the hope a couple of years ago when I first started trying to get RN to "reduce fragmentation" by aligning itself with web, but each small change triggers a slow and costly migration effort. I now think it's unrealistic to expect RNfWeb to become materially smaller in the years ahead, as to do that would first require that most of the existing RN ecosystem migrate away from certain patterns like dynamically reading values from styles at runtime, using non-standard and async events, etc. There's been little progress on that front in OSS over the last 6+ years, and little incentive for us to do that internally at Meta since new cross-platform experiences are being built with RSD.