I agree that as long as the missions work properly, the campaign is a lot of fun. Unfortunately that's not always the case.
So far I have played until mission 7. That's includes skipping missions 5-1, 5-2 and 6. And I got stuck again at mission 7.
As far as I can tell, there are two possible basic problems - trigger activation and unclear mid-flight instructions from Peter.
One example from mission 7 (POSSIBLE SPOILER WARNING!):
After the mortars start firing on x-field, I'm told to take off and locate the mortars (I already had a bad feeling when I heard that - I kinda saw it coming). So I'm following the road back from the airfield, then Peter instructs me to turn left - and that was the end of it. I kept flying for a few minutes before I gave up (having played this campaign this far, I just knew I must have missed a trigger).
I don't know whether it's a trigger related bug or if it's just me, but it sure ain't no fun.
There's a lot of ad-hoc stuff going on during the missions, which can be fun as long as I as the player understand what I'm supposed to do next. But I'm frequently left scratching my head in confusion. With no ability to reconfirm with Peter, the only available course of action left is to quit the mission.