WELL, you can always check out my total war map and see how it can work out ^^^^^ (attachment below)
Some experiences that i could share:
*whenever warcraft recieves a large ammount of movement orders at one time, the motion of units will start to become jagged and the pathfinding starts to become very irregular. Formations are very likely to break up and units risk clustering together. This was the main issue i had and is still negatively affecting the game experience in the map.
*It is very hard to calculate wether a formation is on pathable ground or not. Let's say you order your formation to move to where half of it will be intersecting a cliff, you would have to pre-calculate the formation and it's pathing to judge wether the order should be executed, or if some units can simply be moved a small distance to where it is pathable. . And imagine if, for instance, the formation would be divided by a long wall? half the units would have to go round.
*I have yet to find a way to make it so that the lines would be reformed in a logical way whenever lots of casualties are taken. Right now, i simply browse through the group when i make the formation, since as long as the units ar the same, they will always pop up in the same order. But once one of them dies, the order is completely remade and lots of units have to move to the other side of the formation to get into spot. One ugly fix for this that i have is that whenever a unit dies, the one behind it will step up and take it's place, and the line will only be reformed when 15 units in total have died.