I’ve been using it for a couple weeks but haven’t used RCS, so I can’t say specifically about that. Overall though, it’s still a work in progress and is not as polished but it gets the job done (more or less). If you’re really concerned about privacy using their closed source app, you can just host your own bridges in your Matrix server (the app is the only proprietary part of Beeper, the protocol is just Matrix). The app doesn’t support logging in from another Matrix account, so you’ll have to stick with Element (I think Element derivatives would work too) when using your own bridges. But that’s probably a better option given that their own app lacks a few features.