in fairness heavy duty MTBing brought out the worst in them and they say they have fixed the problem now. I have not heard of reports of such failures very recently.
AFAICT the pawls have two 'ears' on them which the springs locate into. If the freehub bearings are at all slack the pawls can sit at angle when under load and this can (because the pawls were not ground to the right shape to avoid it) load up one of the 'ears' so that it can snap off. Then the broken piece can cause trouble and in addition the pawl can escape its seatings and cause the freewheel to freewheel in neither direction, and perhaps even the freehub body to explode.
If you google 'broken XT freehub' there is lots to horrify you, eg
In most such failures the sprockets hold the mess together and you don't lose drive, so you can ride home perhaps, but the freewheel doesn't work. This means that you can carry on riding but you cannot freewheel, even for a fraction of second. If you have to go far like this, it is probably better to shorten the chain and to run as a singlespeed, otherwise the top run of the chain will go in the spokes if it runs slack, even briefly.
Annoyingly the XT freehub design ought to be stronger (having more, wider pawls, set on a larger diameter) but in reality it is just more likely to break. If there is any slack in the freehub bearings, I think this failure is made far more likely.
cheers
