Premature brushbar failure was a massive problem on the DC24, and many owners seem to report that the brushroll became loud and clattery before total stoppage/burnout, so it could either have been that the brushbar bearings/housings had failed or that the bearings in the motor failed in these instances - I also read that the motors only had a life of 50 hours run time before the carbon brushes wore down, so I think a combination of lack of brushroll maintenance and excessive use could be killing them off too soon.
The other thing with these power heads is that they cut out if they get blocked and people sometimes don't realise they have to press I tiny reset button to get them going again - so they could believe that the head was broken.