Any refractive laser procedure works because it ablates corneal tissue, reshaping and therefore thinning the cornea. This corneal tissue is removed forever, it does not grow back.
A thinner corneal is a weaker cornea and is therefore more susceptible to damage due to trauma.
There have been instances where corneal transplant patients have sneezed causing their cornea to fall out. Incisional refractive techniques such as RK and AK can lead to problems with incisions splitting after treatment. LASIK and PRK have their associated potential problems.
As with all things medical, there are associated risks with refractive surgery. The procedures are far safer these days than ever before, but the practitioners are human, and as you get good plumbers and bad plumbers, so you get dodgy surgeons and excellent surgeons.
regards
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Half man, Half jalfrezi.