Non-union is an extremely serious complication that a LL'er could face and could last for very long time. It could happen with incorrect or careless surgical technique or just bad luck during surgery. There are some studies that show the intake of NSAID painkillers could possibly affect the proper healing of the bones, hence why majority of LL surgeons avoid prescribing them to their patients. Smoking interferes with bone formation. Lack of weight bearing exercises, vitamin D and other nutrients associated could delay union of the bone. I think I read that non-union could also be the result of individual genetics and improper bone healing. Lengthening beyond the recommended length also poses a risk of non -union.
The doctors do bone grafting as a last resort, another massive and costly operation, to try and get a callous formation.
Non-union was the biggest fear during the first 4 weeks of my distraction phase, my doctor told me that if there is no callous formation on the xray roughly after 4 weeks, it could be a sign of non union. The worst case scenario would be amputation of the affected leg.
One more thing, only a person with height neurosis will find this procedure a rational thing to do. If you wanna do LL, go for it, but do so when have fully understood the implications and the risks inherent with LL, with realistic expectations from what you want to achieve after gaining those several cm.