Hail of destruction says it is treated 'like three ordinary shooting attacks (4+ to hit)...'
Since that gives a specific to-hit value I would say that it is not modified by terrain.
Gotcha! by comparison causes the opponent to suffer '3 attacks worked out in the usual way.'
Since that says nothing about the to-hit value, I would treat the attacks as normal and they would be modified by terrain.
On a quick look, Hail of Destruction is the only one of those types of spells that specifies a to-hit value, so I'm probably wrong in my interpretation and the intent of the '(4+ to hit)' is to indicate that the mages don't get the archery bonus- But Assault of Stone then doesn't specify a to-hit roll, so it's weird to have it stated for one and not the other. I'd recommend that all such spells suffer penalties for terrain, but wouldn't argue if someone disagrees on Hail of Destruction.