I'm fairly certain that nobody has had a harmonic balancer problem when they have an updated bolt that's been torqued correctly. Keep in mind that drilling the bolt head for safety wire is only going to weaken it structurally and add stress points from which cracks can develop. If you're that concerned about it coming loose, use threadlocker and torque it properly.

Personally, I'm convinced that 99% of the problem is caused by the bolt not being torqued correctly. This problem is not exclusive to Mitsubishi engines - Toyotas and Mazdas have similar problems - but the engines that have the problems all have timing belts that have to be changed. With an average mechanic possessing questionable abilities or knowledge to do a timing belt job correctly when it requires extra effort to make sure the torque is correct, it's no surprise that they fail.