The way I understand it, there have been two principal methods of vaccination: modified live (attenuated) virus, and killed virus. The live virus vaccine is considered more effective generally, but in small numbers of cases it can become virulent and cause the disease it is supposed to prevent.

Rabies vaccines for cats used to be modified live virus, but some years ago changed to killed virus. At this time the adjuvant was added, which in most cases increases the effectiveness of the vaccine (which otherwise would be less effective than the live virus vaccine). However, the vaccine site sarcomas seem to be associated with the use of adjuvant.

The newer Merial vaccine is live virus (as I understand it -- but I'm not an MD or DVM), but is recombinant and this is supposed to make it unable to become virulent. Because it's live, it should be more effective, and therefore not require adjuvant.

Other than this, I can't judge what other risks may be associated with it.