It means that library maintainers are having to make changes to their libraries to get them to work on the serverside, and a lot of them aren't not really doing that very quickly.Consequently moving to RSC reduces the number of libraries that work with your React code.
It's a short term problem because most popular libraries will get updated eventually, but some won't and they'll only ever work on the client side.
In the case of Next, maintainers need to package their libraries differently to support ESM modules, or you need to configure your project to use the experimental.esmModules=false flag. Again, it's not a particularly big problem but it does reduce the size of the available ecosystem a bit.
Before, there was only client-side. Server-side is opt-in. So all the libraries you used client-side, you can still use client-side by using client-side components
It's a short term problem because most popular libraries will get updated eventually, but some won't and they'll only ever work on the client side.
In the case of Next, maintainers need to package their libraries differently to support ESM modules, or you need to configure your project to use the experimental.esmModules=false flag. Again, it's not a particularly big problem but it does reduce the size of the available ecosystem a bit.