Hey, I've discovered your campaign just recently. If I've understood well, there are 4 missions ready to play and one other ready, but not published.
I've only played the first mission yesterday, in brutal. I didn't play the other difficulties, so I can't tell if it's well balanced, but it's not always easy in brutal.
However I have certain things to suggest/says :
First : the moment of the first attack on the zerg base to rescue lost soldiers is a lot borring : it takes a long time to destroy the building for nothing. You could maybe erase one building, sumone zerg reinforcement from larvas, or simply make the lost army join your side when you encounter them (and it would be more logical as well) even if you didn't destroyed the building. If you go on this way you could prevent any rush by puting a special unit that would be doing a lot of damage in one go (with a spell) in a way that no rush would be done. Either way, you could simply find a way to destroy theese buildings faster. In a second time, in the same area, the lost medics follows you if you are hurted, but the others units would stay there, unmoving, it's completly irrational.
I would suggest also, to improve the difficulty from the moment you are given the lost soldiers, to the moment you encounter the blue guy (I don't remember his name), I was in brutal, and it was a lot more easiest to do this part, than the last one when I had to defend the base.
Another suggestion : implement automatic saves in key moment.
And I believe you already work on it, but the tank don't shoot at the rock direction :/
The last thing I found is, energy regain while the cinematics, so if you skeep them, you will be a lot less strong than if you don't. I believe there is something like a pause option for units in the map (in the trigger) that stop every movements, attacks, producing, regeneration, etc.
In other way, it's so beautiful :O The map and cinematics are so awesome, it blowed my mind. I like your first map. Hope you will work as much for the others. It looks like blizzard work.