This type of locking wheel nut is generally quite sturdy and reliable. Most problems with these come down to a lost key rather than damage to the locking wheel nut itself, although rounded or damaged keys are still possible.
Type 1 locking wheel nuts use an “anti-tamper” head that will snap off very easily, leaving only a flat surface behind
As this is the older Ford style locking wheel nut, failures are usually caused by corrosion rather than the bolt itself being weak. They often develop an “over-tightened” feel, even when they haven’t actually been massively over-torqued. Snapped or rounded locking wheel nut keys are much more common with this type, especially if force is repeatedly applied once corrosion has started.
Type 1 locking wheel nuts use an “anti-tamper” head that will snap off very easily, leaving only a flat surface behind
Type 2 & 3 use spinning rings designed specifically to prevent this kind of removal. These rings rotate freely, making it much harder to grip the locking wheel nut with sockets or extractors.
Don't Force It:
Attempting to use a broken or damaged locking wheel nut key is unlikely to remove Ford locking wheel nuts. once the key is damaged it will just spin and the wheel bolt will no longer engage correctly.
Locking wheel bolts are generally made from much harder steel than mild steel. Poor drilling technique or the wrong tools can cause the bolt to work harden, making removal almost impossible without professional grade tooling.
Chisels:
Hammering a chisel into the bolt might seem like a good idea, but the chance of successful removal is low and the risk of damaging the alloy is very high. Once the wheel is gouged, the repair can end up costing far more than the original problem.