Yes, the Alt move isn't all that predictable sometimes. First, you have to guess where the exact center of your battle line is, then click ahead of that center... otherwise your army turns in a direction you don't want. Sometimes I'll need several Alt-right button clicks to get them moving forward without turning.

The army can get disrupted during movement by uneven terrain that slows down some units more than others, so the whole formation falls apart until they get re-organized at the endpoint of the move. The Alt-move seems to be much more predictable on completely flat terrain with no vegetation.

And finally, just as the individual units don't "know" that parts of the line are being slowed down by terrain, they also don't adjust unit movement speed, to keep the battle line moving uniformly, at the speed of the slowest unit. The cavalry walks faster than infantry, so they'll usually end up arriving at their position before the rest of the army... the general on horseback will walk right through the front line infantry while moving, etc.

So what I usually do is a series of small Alt-moves, sometimes with a tweak using the "," or "." keys to rotate the army if needed before the next move. And mainly it's for positioning before combat. Once we're in contact with the enemy, I'm usually selecting and commanding individual units or unit groups, because the Alt-move is too risky. The only time I'll use it after battle is joined, is when I want a fast way to call back skirmishers and cavalry to the original formation after successfully driving off an initial attack. That can be useful, combined with the R (run) key... as long as I'm careful not to have returning units exposing their rear to enemies who are too close.