Q&A
By reforming you change your religion, so you loose some properties and gain some others. It is up to you to remain "uncivilized" if you prefer a life of looting... As an unreformed religion, if you have a strong leader, the tribal politics modifier are quite good. However a reformed religion is overall more stable, and the difference should be clear for young, unproven rulers who will have an easier time. With the build presently publicly released, reformed religions are not really coded though, so it may not be worth it reforming at all. We are just coding in now the reformed religions.
Stay with us a bit more and everything will be more clearly documented.
Proselyte religions mostly impact attitude for conversion. Some religions don't become proselyte even when reforming anyway. You should look at the propoerties of your reformed religion to know that (in the Matrix).