I have only had a few that developed a lump at the injection site. I have never had one develop into a cancer or that got any bigger than a small marble. Every one of them went away after a few weeks at most.
I have however decided to stay with one particular manufacturer. I did some studying and found this company to have the least amount of vaccine reactions. If I did not have so many cats, I would probably switch to an every 3 year rccp rather than once a year to reduce the risks.