How many times does a horse walk up and grab an electric wire? Most horses do it once. Running into a nail is the exact same thing.
As I've said, I am sure the nail thing works. However I've never had a horse who did not realize that getting in my space was un-comfortable for them and thus I've never had to bring sharp objects into my training routine. It is only ineffective if you do it incorrectly. I have no head shy horses here. I do have horses who hop aside when my arm goes up, which comes in handy when I walk 5 buckets of grain out to 5 horses in the same lot and they wait patiently for each bucket. The response is simply what they'd do to each other, if done correctly they catch on rather quick and it beats a nail/eye mishap any day (which would be a huge downside, IMO) again, I did not say it doesn't work or tell the OP not to do it, I offered the opinion of what I do.