There are definitely lots of factors that aren't considered at all by the difficult/easy/square/glancing/etc strikes; you basically have to know your enemy.
You may be able to frequently land tricky or even difficult strikes on weak enough enemies (good to know for quickly decapitating hordes of kobolds and such), whereas some enemies have such high dodge you learn not to waste time on anything worse than "easy."
Armor matters a great deal, even a "very square" strike will often harmlessly glance off a decent helm, but if limbs aren't armored you can lop them off even with a "can't quite connect" attack, so be sure to examine humanoids and check for armor.
Large enemies shrug off most attacks no matter how squarely you hit them, the fat and muscle blocks most of it; aim for the head if you can and upper/lower body if not, hacking at their limbs just doesn't seem to do anything. Whereas normal-sized enemies can be significantly weakened by severing a limb, if you get a good shot do so and it'll make future attacks on the body/head easier to land.
Take any opportunity strike you ever get against the head; I've even seen an opportunity scratch hit the brain and instantly kill.
Otherwise I ignore most opportunity strikes (an opportunity kick or punch on the body/limb just doesn't seem to hurt as much as a regular axe slash, and nothing you do to an opponent's toe will have much effect) unless I just can't hit with anything and get desperate.
Anyway those are my observations/best guesses, I may be wrong; as I said, the UI just isn't taking most of these things into account, you mostly learn by trial and error death Fun.