L18_B110,
The only time I have herd of these things not working as a locker/unlocker is when the production tollerances are not exactly suited to the diff centre or the torque they have to take is too great. They can sheer the diff centre drive pin if the torque loadings are too high resulting if complete diff failure.
I have a high powered 1397cc/stroked/efi/close ratio gearbox etc etc that is puts out heaps more grunt that a stock A12 but again I say that the limiting factor to the amount of grunt I can put to the ground is the road surface and tyres - in my case dunlops in the dirt. Things might be different if I were competing in hillclimbs etc.
I have herd of other failures like the diffs remaining locked and not unlocking or the diffs remaining unlocked and not locking but I believe it all has to do with accurately designing and fitting the spring loader locker plates to each and every individul diff centre.
I have spent a long time getting all this right and I'm glad I did or I might be one of those production customers that has nothing good to say about them. All I know is that I'm glad I spent the time doing mine right because it is doing a better job than I thought it would