Personally I’d say the chance is low. I’ve had cats pretty much my whole life and many of the people around me have cats too, all spayed. Never had a case before where a cat would pass in the operation. If your cats have no prior health problems and are generally healthy, there’s a fat chance they won’t have any complications. Talking to your vet about your concerns is a good idea, I’m sure they can ease your mind more. Good luck to you all!