The dangers of running nightlies are described in the FAQ
Some of these problems will also affect the v5.0.0-RCx releases. (The libs for example will most definitely still change. The documentation is not yet done. …)
The main issue of using nightlies in production is that they are normally not guaranteed to produce files that can be opened by different versions. All files created by a release candidate should be compatible with the eventual version 5 release. (Unless a really bad bug has slipped through until now.)