Is it true that laser vision correction surgery is safer for high myopia than for “low” defects up to -4 diopters?

This is not true. The safety of vision correction surgery does not depend on the size of the defect. It is constant, because the technology of performing the procedure is the same, for different defects. However, the effects of the procedure depend on the defect, because it is easier to correct a small or medium defect than high myopia.