In general I would say Masters Races have fewer crashes than 4/5 races around here. However, I've been in some Masters Races here with a lot of crashes. One factor is that the Master 45 and Masters 35 races can be the biggest fields, often bigger than the Cat 5's, and the more people you bunch up on a tight crit course, or single lane RR, the more potential for crashes.
Also, you better be a prettty strong Cat 4 to hang in the Masters 45 races, and a strong Cat 3 in the 35's races.