Just because on your strategic map you have placed 3 of your stacks around the enemy before starting the battle with one of them, doesn't mean that in the real timing of events within the time span of that 'turn', everything turned out to be so nicely coordinated and all 3 of armies were allowed by the enemy to coordinate the attack at their leisure.

I simply imagine that all my 3 armies are not separate; they all march together, if only there was a "link army" option. I get the need for a 20 unit cap though.

Reinforcements simulate friendly armies rushing to the battlefield as well and as soon as they are able - you don't get to choose which friendly unit makes it there first as it's a 'best effort + luck' kind of thing.

I get your point, but the game doesn't allow me to march my armies together. If they were separate what you said would be valid.

in this too you are forgetting that this is a simulation of warfare and sometimes just because you order your troops to do something, doesn't mean they will be able to execute that order successfully.

Unfortunately, no matter if my units are the most disciplined, unbreakable badasses there are, they just don't listen. Also, this is a game. When you click on an army and tell them to go somewhere, they do so immediately and exactly. Should they have a delay? Should they sometimes just get miscommunicated? No, because this is a game. So they should retreat normally as well.

In our game all these potential mishaps are simulated via 'sometimes enough models get stuck in melee that the regiment can't properly disengage'

If they are surrounded I get it, there's nowhere to run, but if there's noone behind them or they're a big unit that can charge through everything, then just get out of there.

As a good tactician you are supposed to plan for such failures - if you watch some battles online for example you will see some players taking the care to engage an enemy unit by another from a flank or rear or disrupt their ranks with a chariot charge just to allow another crucial friendly regiment to safely disengage at far higher probability of success.

That would help. But as a good tactician I can't plan for "bugs". You see I click retreat, but they get caught up one time and totally give up. I must click retreat again. They should keep trying to get out, but they don't. They entirely cancel my order.