resume works best when you limit the availability of extra cars. rF2 randomly selects cars from the list you click on, but if you choose 30 cars when 45 cars are available, the possibility of a mismatch occurs when rF2 randomly selects cars to populate the replay. (That doesn't read well, sorry) I believe rF2 treats a saved replay just like a race and follows the same loading of cars, the random loading of cars, as if it were a stand-alone-race. If one of the 15 cars you excluded is chosen by the sim for the replay, it may crash or cause skin mis-matches. Add one other caveat...S397 updates cars, often with very little warning. If you start a race with v1.07 car. And Steam uploads v1.08 to your system, I'm not sure the replay will still work. Perhaps others can fill in more.
Gringo's correct, you CAN do it, but like all things rF2, it's often not simple.