You are right with everything you've said. I think most people think that the risk from the actual operation is much less than the risk of cancer. 80% of female rabbits develop uterine cancer by the age of five where as the risk of spaying is something like 2%.
HOWEVER...
The risk depends on the vet doing the operation. On this forum we like to find what we call rabbit savvy vets. Rabbits are classed as exotic pets although most people don't know this. Crazy as they're England's third most popular pet. So you need to find a vet who knows about the needs and health of rabbits and the risk should be greatly reduced. You can ask your vet lots of questions, things like how many female rabbits have they spayed and how many have been successful. 100% of them should have been successful!
Also it might be worth making a thread on the forum here in rabbit chat saying something like 'looking for a good rabbit vet in [your area]'. Someone might be able to suggest a good vet that they use.
Unfortunately there can still be complications but they are usually due to underlying health issues your rabbit already has and can't be avoided. But I think almost everyone on this forum would say you are making the right decision getting them both spayed and neutered. For males it makes them much more relaxed and for females not only does it reduce the risk of cancer, but when female rabbits get to sexual maturity they can get very vicious and territorial and spaying helps to stop this. You'll be happier with two calmer bunnies who will hopefully be a loved up couple.
By the way, where did you buy your rabbits from? If it's a well known pet store chain and they sexed them wrong I would complain. Sometimes they will pay for the neutering for you which is obviously worth thinking about. :wave: