The risk is small, for a healthy rabbit with a rabbit savvy vet, HRS has had a 1% mortality rate due from anasthesia. I have no idea why your vet would say the cancer risk is not high, given my vet treating rabbits for 25 years agrees with the cancer risk being too high in a female to leave her undone unless there is a reason and HRS also lists it as a reason to spay (he is OK not neutering a male if no reason to, since he hasn’t had many cancer issues in males). Not to mention, an unspayed female is hormonal and territorial often and can get false pregnancy, especially with a male rabbit in the house, leading her to believe she is pregnant to the point of actually building a nest (plucking her own fur out to do it). If you’ve ever seen a female in false pregnancy mode, you would not want to see it again. :s
If you are interested in the scientific study they did, where the 80% number came from here is an article on it: http://www.houserabbit.co.uk/resources/content/info-sheets/uterine.htm. That also lists the NAME of the study, which if you do a google search you can likely find the original study if you want to read that, but it will read like … well, a scientific journal.