My MAIN issue with the auto calc is that it takes casualties from apparently random units.

My elite archers, whom I would never allow into a melee, lose half their number, but the unit of raw peasants (who would almost certainly be used as an arrow magnet) is almost totally untouched?

Duabayew tee eff, mate.

And, as mentioned, and as I have said before, the seemingly random chance of "LOL DEVASTATING DEFEAT" under strange circumstances is not a risk I'm willing to take with a large stack.

It's fine with bandit/rebel/rouge AI stack hunters, since I rarely put more than a couple cheap units in there and can easily rebuild them. But for my two stack army, six provinces from a place capable of sending reinforcements, I avoid autocalc whenever possible.