Easy question, but not easy answer. Anytime an animal is put under anesthesia, no matter the age, there is a risk. Does she have any pre-existing conditions with liver, kidneys, obesity? The spay can be much more difficult with age over 2 years in a large breed dog such as this. Many things are possible whenever any animal or person goes under anesthesia. Your vet is making you aware of this. At your dog's age it would be advisable to do some blood work prior to surgery to rule out any liver or kidney problems. If you think the warning is due to lack of confidence then seek care elsewhere, otherwise take the warnings as just that. I have been a vet for 21 years, anything can and does happen, we can only do our best to avoid it. Complications are rare indeed, but some people are unaware risks exist. Keep in mind this is done under general anesthesia (preferrably gas) and is major abdominal surgery. Having your dog spayed is a good thing at any age, but much preferred to be done under 1 year of age. Hope this helps.