Lots of factors influence whether hybrids are formed. Basic chromosome homology, number of chromosomes, etc. all influence this. Unfortunately the classical taxonomists got a lot of the relationships wrong. For example, we now know that Aeoniums, which were once thought of being closely related to semps, are not at all.
Sedum is such a diverse genus and it is now clear it's really 5 or 6 different genera with similar morphology. They do seem to be a bit more promiscuous than many other succulents. Rowley has a wonderful book on hybridizing succulents that cover many of these issues.
Kevin
