As they are older, it can be a bit more risky. You should get blood work done before they go in for surgery to make sure they are healthy, most vets will require an exam before the surgery anyway so this can be done at the same time. Older rabbits can also take a bit longer to recover and heal.
It is very important to go to a rabbit savvy vet. They know how to care for rabbits and the specifics that a dog/cat vet will not know. For example, dogs and cats need to be fasted before surgery as they can vomit and choke on it. A rabbit cannot vomit so can and should have food up until they go in for surgery. Sometimes a receptionist or tech will tell you to fast the rabbit, if this happens you should talk to the vet, if the vet says to fast you should find another vet. Rabbits should also have food avaliable right when they wake up so they can keep the GI tract moving and avoid GI stasis. If you can find a good vet, the surgery should go much easier on your and the rabbits.
Any surgery has risks. A good vet who knows the species and what they are doing will help lower the risks by a lot. Getting blood work done will also help detect any issues that could also cause problems. You should ask the vet how many rabbits they see in a week, how many spays they have done and how many rabbits they have lose on the table. If they don't see many rabbits, don't do a lot of spays and have lost more than 5% of the rabbits, then you should find another vet.
