I can think of one big difficulty from the start.
You deprive him from the VP for that building, but by doing so, you also deprive yourself of them. Read the rules for Orbital Strike. with bad luck you will kill yourself if you try to take that building.
It can hamper your enemy enormously tho. But only for a round or so.
I don't have the new codex yet so i don't know how the cityfight rules are, but i would classify the idea as moderate to good if there are a lot of buildings that give VP's. the effectiveness is best imo when there are only a few buildings, as with more the enemy can afford to leave one alone.

 

Actually an OS fires every round of the game from the round it starts. So once it starts firing the target area cannot be entered by anybody without a deathwish or great confidence in invulnerable saves.

The idea is that you deny your opponent a building you probably couldn't have taken anyway. In most cleanse missions unless you wipe out your opponent you rarely get all four table quarters, and it's the same thing here, you deny that building in his deployment zone that gives great lines of fire for his devastators and which is too far away for you to have much chance of capturing.

 

my fave is using it on vital destructable buildings, there nothin funnier in the new cities of death than letting your opponent get confident pile unit after unit into it thinking he will get mega vp points then blasting the entire thing occupants and all into oblivion and him gettin nothin ^_^

 

Instead of aiming at an important building consider aiming where he will go. This way you can use the orbital to mark certain areas so it acts like an explosive wall.

This is also very viable for retributors, a squad of heavy bolters could be used to mark the length of a raod.