There haven't been any betas or anything else for Wii since the last update. The code is all up on GitHub, so users can compile it (and share it) if they want to, which is what happened. It wasn't meant for any particular type of user, it was just a build of what code was like at the time (highly transitional, i.e. unstable). It showcased a couple of new features like SuperrSonic's work adding "Set Screen Width" so that users can more accurately capture the aspect ratios of their original consoles, but also has relatively severe issues with both input and display.
Overall, it's usable but highly inconvenient: Wiimotes desynch when launching a game or changing cores, RGUI will only display sensibly when running at 640*480 (wrong resolution for most platforms emulated by RetroArch), so doing something like changing a setting becomes a many-step process of restoring the resolution to 640*480, navigating to and changing the desired menu option, navigating back to video options to put the resolution back and returning to the game.
If you don't have a pressing need to switch to gain a new feature or two, you're probably much better off sticking with the stable version. Personally, I use the newer Gambatte core for the benefit of Set Screen Width since GB/C are otherwise extremely difficult to get square pixels in on the Wii, but if you plan on changing settings, reloading games, etc. frequently it's no fun. Pick a game and stick with it and the RGUI problems mostly fade into the background.