Some people should definitely not have excimer laser surgery. They have no need for the procedure. People under 18 years old. Their nearsightedness is probably still increasing. People whose refraction is significantly changing.
Refractions often continue to change through the teenage years into the early twenties.
At least two years should pass without a significant change. People who insist upon a perfect correction. A perfect correction is possible but cannot be guaranteed. Women who are pregnant or who are breast feeding. Hormonal changes will often cause temporary changes in your nearsightedness.