A lot of people reckon they’re better- tested to higher standards etc, capable of withstanding greater impacts and tested as such. The pushbike CEN is pretty undemanding after all.
Not really convinced myself, they’re designed with different agendas in mind. The venting alone is a big deal- keeping cool and comfortable is good primary safety, helps you avoid crashing in the first place!
But DH helmets are generally lighter (comparing like for like) and can also be “softer”- both quite a big deal in terms of injury reduction. Weight obviously puts more strain on the c-spine but also increases dwell time of the impact forces, whereas a less resistant helmet will reduce the forces on the brain since it decelerates slower- up til the point that it’s overwhelmed.
One other thing to remember is that a helmet that never cracks, crushes or otherwise breaks is too strong- it’s sacrificial protection. So seeing a helmet crack or break isn’t automatically a worry, it just means it’s laid down its life to protect yours. If it happens too easily, then that’s a problem.
The venting alone swings it for me but tbh if I had a moto-x helmet I wouldn’t be against using it, I just wouldn’t choose to buy one over a dh helmet.