@red3 Ruth Ann, that's a great question. I think there are many surgeons who will only do spine procedures with the insurance that hardware instrumentation gives them. Hardware does increase spinal fusion rates, but isn't always required. They used to do cervical fusions without hardware as a normal procedure, but after the hardware and plates became available, it gave surgeons a chance toward better statistics. Non-fusions can happen even with hardware, and then it is the hardware alone that is holding it together and that can fail. There can also be problems with hardware if screws become dislodged or if the implant becomes displaced or if there is an immune reaction to it. Here is some literature that I found.

Misplaced screws
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5400184/
Here is a lab that tests for implant material sensitivities. I had these tests done, but then opted for no hardware instead because a patient can develop an immune problem later even when testing prior to surgery indicated no issues.

Orthopedic Analysis
https://www.orthopedicanalysis.com/
My surgeon would do this surgery without hardware because it was only one level; if I needed more, I'm not sure what the answer would have been. I know someone who did have 2 levels done at the same time without hardware, and only a bone graft and neck brace, and she did fine and has no issues even 15 years later.

My surgeon said a fusion shrinks as it heals, and the length of a plate on the front of the spine can cause issues if it is too long and contacts the adjacent disc which would lead to damage and adjacent segment disease. My surgeon had to trust me to be careful to not screw up the work he had done during my recovery. I promised to stay 3 months in the neck brace, and it did take that long for the fusion to begin. The bone graft spacer is also tight enough that he could tug on it during surgery and it didn't move and the bone surfaces would be rough because the bone spacer is milled to a specific height before he gets it, and the prep during surgery is to grind the end plates on the vertebrae to have a straight surface so the spacer will fit properly. There is also tension in the neck muscles holding the spine, and I had to avoid any bending or twisting movements. You get used to doing things without looking down or turning your head.

Your health also affects fusion rates, and I did everything to optimize my health before my spine surgery. I took multivitamins. I had my thyroid function tested and made sure my dose of thyroid hormone was correct because I have a thyroid condition. My vitamin D3 levels were tested and optimized. I am also on bio-identical hormone replacement which helps with bone healing by essentially turning back the "aging clock" just a little bit. I don't smoke or drink. I walked for exercise to get my body's available oxygen levels higher. I ate a healthy diet with enough protein. The bone graft also had a space in it to accept the bone spurs that were removed during surgery, and those seeded bone growth for healing. My body had been doing a good job growing the spurs in a bad place, and it was good to get them to do something helpful and in the right place.

I found this literature that describes and compares procedures with and without instrumentation if you read the section on "Anterior Cervical Discectomy with Fusion" and it shows some imaging for comparison.
http://www.neurologyindia.com/article.asp?issn=0028-3886;year=2012;volume=60;issue=2;spage=201;epage=209;aulast=Muthukumar
Another study
https://www.ncbi.nlm.nih.gov/pubmed/17414911
Here is a guide from Vanderbuilt Health that also mentions cervical fusion without instrumentation.
https://prd-medweb-cdn.s3.amazonaws.com/documents/spinecenter/files/anterior-cervical-fusion-guide-01-2019.pdf