Procyon Insurgency definitely had a few missions that were a tough nut to crack, but i have to say that i quite enjoyed cracking them.
If it gets too frustrating, one can always switch to "very easy" as a last resort, but i would really feel that making these missions easier as a default, would make this campaign less unique and hurt it's mood and atmosphere in the process. There's nothing wrong with the occasional "tough as nails" campaign. If anything, then variety in campaign concepts is a good thing in my eyes and certainly not all campaigns need to be built around blasting through every mission on first or at least 2nd try.
Yes, this is something that I think gives PI a bit of a unique flavor. I haven't gotten around to playing some of the recent releases, but many other campaigns I played in the past were too easy to the point that they essentially became interactive stories, and you could pass most missions on one try without having to think much about how to approach them. The whole gameplay and tactical planning aspect of FS2 is something that I think was only well developed in the multiplayer missions, and I wanted to bring some of that into a singleplayer campaign.
As far as mission 15 goes, at least the base objective of 15 turrets is not too hard. There are various long range weapons you can use that let you sit outside most of the Saturn turrets' range (Akheton, Maxim, Trebuchets and Stilettos all work), so it's only the fighters that pose a threat.
And I just don't like the mechanic of disarming capital ships from long range. It makes these enormous, romantic warships seem so toothless and helpless. They should be summoning friendly fighters from elsewhere in the system.
I agree with this to some extent, and in that mission I tried to give the impression that you were creating chinks in its defenses without dealing any sort of crippling blow, as the ship was too big and powerful for that. Getting too close to the ship means a quick death, the main beams cannot be affected and you don't realistically have time to get more than 25 (out of 66) turrets before it leaves. In a sense, the relatively high difficulty also suits the situation.