To clarify a few of my points in response to your post (I love talking about balance
In regards to AI usage in missions, I think the AI class being used is more an indicator of how experienced the FREDder is, i.e. a beginner doesn't necessarily know how much influence s/he can have on mission difficulty by simply tweaking the AI class. The number of enemies could also be interpreted that way - if you don't realise how effective changing the AI class can be, simply adding units is a quick and easy way to tweak the difficulty. Also of note is that in the olden days of FSO, before AI improvements were added, adding tons of units was pretty much the only way to create a challenge for the more experienced players (I'm looking at you Derelict). Bottom line is, I'm not sure how reliable these metrics are (of course, it's possible I'm projecting waay too much of my personal experience into this :/).
Regarding my definition of balance with regards to difficulty, to evaluate whether a mission has been balanced for insane, what's really important is how much that difficulty evolves in relation to the baseline easy/medium. Basically, I think difficulty should be consistent accross difficulty levels. FS2 contains some good examples of that :
- It is usually agreed upon that the most difficult missions on insane are the ones from early in the campaign, where you can't give your wingmen harpoons or are forced to do extensive dogfighting in a heavy fighter.
A specific example : Slaying Ravana : On easy, it's a fairly challenging mission but your bomber is beefy enough - thanks to the difficulty's buffs to the player, to resist what few turrets & fighters can shoot at you, plus your wingmen provide an adequate diversion before they go down. On insane, your wingmen get vaporised within the first minute or so and you are left having to deal with every hostile fighters and turrets trying to kill you while attempting to throw bombs at the Ravana (who also tends to fire at you through its own hull).
- On the other end of the spectrum, a good number of missions get easier due to your more competent wingmen and friendly warships firing faster, such as the last stretch of the campaign when dealing with massive waves of bombers. In fact, I think insane is the only difficulty where it is possible for the Aquitaine to destroy the Moloch attacking it in the two missions where you escort it out of the nebula.
I'll admit it's been a while since I've played ASW, so my memory may be a bit off, but I seem to recall a few missions that rely on throwing massive numbers of fighters on the battlefield, which usually means that difficulty is going to swing sharply towards one end or the other.
The old Wings of Dawn is a more subtle example in that most missions are going to be consistently tougher than in easy mode. However there are a couple of missions that are practically impossible, and Cordi fighters get ludicrously dangerous.