Many of the boarders seem to like the "half shell" helmets for the park. Just My HO ... they would be ok for the lower areas on the hill out of the wind and less speed involved in the relatively "protected" areas of the terrain park.
Issue #2 would be full coverage protection versus that of a half shell.
Issue #3 ... the cross and park helmets often have a visor. In a higher speed tumble, that looks like a good way to wrench your neck in a rolling fall, similiar to the issues of why a chin guard is OK for slalom but not a good idea for speed events.
The only drawback to a full coverage helmet is that I miss hearing the sound of the snow under my feet. Here in the east where the hardpack and ice are the norm, that is an issue. As a "soft snow" westerner ... possibly less so.