There are many different ways to fuse the spine depending on the spinal level of concern, surgeon experience and expertise, need for instrumentation (e.g., metal plates, cages, rods, screws), and patient size (obesity makes an anterior approach more difficult).
Studies have been done comparing different approaches (e.g., anterior, posterior, transforaminal, interbody). While each one has its advantages and disadvantages, the overall results seem to be similar across the board. Complications can occur in any surgery but spinal fusion is by and large quite successful.
The area you mentioned (L5S1) can present a challenge because the pelvic bone (ilium) is so large and obstructs the area from the side. Some surgeons are now using what’s called a transaxial approach to fusing L5S1. A lateral incision is made and the surgeon goes through the psoas muscle (hip flexor along the front of the spine). The disc is removed from this vantage point and the spine fused using a minimally invasive system called axial lumbar interbody fusion or AxiaLIF.
There are some technical challenges and complications reported but it should be noted that not everyone experiences these. Injury is possible to the group of nerves or blood vessels to the pelvis and legs. Damage could occur to any of the muscles that are transected in order to get to the spine. Most problems are temporary and last less than a month. Occasionally, a patient will have a symptom that lasts as much as a year but this is unusual.
After reviewing many studies on lumbar fusion from 1950 to 2010, it is clear that more studies are needed to directly compare different fusion methods. There is not enough evidence at this point to say one approach is superior to the others. Tissue engineering and tissue regeneration may eventually replace surgical fusion. Such advanced biologic techniques could eliminate nerve or other soft tissue damage caused by currently used surgical methods.