Other than weight, what are the disadvantages of using a helmet designed for dirt biking rather than one designed specifically for mountain biking?

Obviously a helmet made for dirt biking or motor biking is going to be a lot heavier than a dedicated mountain biking full face helmet, and it'll likely be a lot worse in terms of ventilation too since it'd be designed for higher speeds and for a lower level of physical demand from the rider, but are there any other considerations that might make it impractical as well? And how severe are the disadvantages in practice for a casual rider?