There is always a risk with any surgery that requires full anesthetia. The risk is not greater for bunnies than for other animals or humans if the vet is bunny savvy and has the right equipment to anesthetize rabbits. Most of us here have had our bunnies neutered or spayed, so there are lots and lots of people who’s rabbits have had surgery.
I have no experience in having a lump removed from a bunny, just from a dog. They take out the whole lump and send it for testing, because if you only take a sample of the lump and if it is cancer, the sampling may trigger metastazising (the cancer starts to spread by sending out cancer-cells into the blood-stream).
Post-op care for rabbits includes getting the bunny to eat sth asap, so keep some yummy stuff at home and send some yummy stuff with him to the vet so he can eat as soon as he wakes up if he wants to. Water should be served in an open dish because a bottle may be bothersome for the rabbit to use. The bunny needs to be kept warm, they can suffer hypothermia after surgery and that is dangerous. A hot water-bottle in the cage with a blankie around it and enough space so the bunny can move away from it if he pleases, is good.
I hope it’s not a cancerous tumor, but it must be removed for the vet to be able to tell.