Good point, and very hard to find the real stats on this. The private clinics reckon the failure rate is about 1 in 1000. The British Journal of Ophthalmology puts the figure at 1 in 10.
I have a feeling the failure rate is less than one in twenty these days. Also one has to define "failure". I guess one can call my wife's 1st go around a failure as she didn't hit the exact amount of required correction but the second time she did.
Another good point. 'Failure' is defined as 'it didn't work', which doesn't necessarily mean that the recipient was harmed in any way. However there are rare cases of blindness, less rare cases of vision becoming worse, and quite a few cases of night vision becoming so impaired that the patient can no longer drive in the dark.
The most common problem is 'dry eyes' which seems to happen a lot, but can be corrected apparently.