In MTW auto-calc is actually useful compared to the human played battle. AKA it don't cost you an arm and a leg, unless even the battle you played played cost you atleast an arm.

You get your initial 16 units and every unit over that is given as reinforcements, that is basically unlimited (had once over 1000 units as reinforcements, required more than the entire map to support that army though) so the battles can be more epic and decisive. One big battle can break the neck of an empire for decades. 3000 vs 5000 isn't an unusual battle size (when one stack = 960 men).

The downside is that the enemy general often dies as even the rearguard was killed (he often joins the fray as one of the last units), weakening the AI that already have problems with doing well with his reinforcements.

The victories are often way less loopsided.

In both games the AI cheats a bit in the 2 higher levels. Nothing really major. Only on the battlefield in MTW though.

Every faction have morale improving buildings (it was the religious buildings). It's good as it makes the troops more reliable, but you still had to guard them as they will still rout if things went bad.

The terrain features on the MTW battle map was affecting tactics much more.

The difference between running and walking was less in MTW. Thus walking was more useful.

Cav actually get tired in MTW. And cav charges are less powerful. A frontal charge is only lethal vs very weak units (or with very strong cav). A flanking charge is another matter though. Flooding with multiple cav with frontal charges doesn't work well.

On the more downside stuff in MTW, here's some examples
Generals (often kings) can occationally have alot of hitpoints and valour and as only 2 men can engage the same enemy at the same time, those generals could last forever and killing up to hundreds (claimed, "only" seen dozens myslef) of enemies before fleeing or dieing. They are commonly known as jedi-generals.

With a massive trade-network and with some time, money never gets a problem in MTW, sadly the AI isn't as good on this. Modable fix on this though and I pesonally actually prefere this, but it isn't balanced and weakens the challange.