It is absolutely understandable, and I think any owner who has had their rabbit spayed or neutered has the same reservation until their rabbit returns home safely. Thankfully, spays are much more common now among rabbit vets, so as long as your rabbit is healthy and you find a rabbit-vet who is experienced with doing successful spays (not all rabbit-vets do spays often, so asking is a great idea), your rabbit will most likely make it through A-OK. It's also good to keep in mind that online, people are more likely to post when things go wrong (i.e. the spay leads to an unfortunate passing) than when things go right (i.e. the spay when smoothly and nothing of note happened), so reading through those posts can give a false perspective on how often those complications actually happen. I also think that in many of those cases, the passings can unfortunately be tracked back to an inexperienced vet or an underlying medical condition not discovered until the operation. There are exceptions, but again, if your rabbit is healthy and you find an experienced vet to do the procedure, it's relatively safe. Most vets wait until the rabbit is 6mo to spay, due to developmental reasons.
There are also great benefits to having a female spayed. In addition to reducing those territorial markings, it can also reduce any territorial/destructive behaviors that can be rooted in rabbits' natural hormones, such as being defensive/aggressive about territory. Also, if you eventually want to get a friend for her, she will need to be spayed for that to happen. Females can also experience false pregnancies if they are not spayed, and from what I've heard, this can be frustrating for both the owner and rabbit. There's nesting and fur pulling involved. I've heard it doesn't happen too often (though it varies), but that's something to keep in mind as well. Most prominently, spaying prevents the development of uterine cancer in females, which has a very high chance of developing by the age of 4. Many rabbits go through emergency spays when this is discovered, so it's a good preventative measure. The likelihood of uterine cancer varies depending where you look, but percentages go from 60% to 80% chance of a 3-4 year old female rabbit to develop uterine cancer.