I'd be surprised if we were to actually hear of any structural lock failures. The plunge lock is, as you've surmised, a very strong locking mechanism. To structurally defeat it, you would basically have to shear the lock button itself, a feat that would require a great deal of force.
I would say that a far bigger potential problem with the plunge lock is the fear of accidental lock disengagement. I use my L-UDT quite a bit in various carpentry projects, largely because the lock strength instills so much confidence. But occasionally, when I'm reaching around in an awkward manner trying to trim off the dried remains of some excess polyurethane glue, I can feel a thumb or finger work its way onto the lock button. I haven't actually ever depressed it far enough to cause the blade to disengage, but the possibility of this happening has crossed my mind. MOD actually developed a secondary back-up slide safety on their CQD model which compliments the plunge lock and further prevents this type of accidentally disengagement.
