With something easily visible like README.cmake containing appropriate disclaimers?*

I suspect that build bugs are likely to be found more quickly with the stable branches just because it fits into more applications and/or workflows (i.e., more people).

*i.e., probably has bugs in the build, has known limitations (see list), build interface may change in a future stable release (until it is no longer experimental), etc.

Born in Roswell... married an alien...