The Undead don't know when to hold 'em, don't know when to fold 'em, don't know when to walk way, and don't know when to run.
Human soldiers fleeing a battlefield is usually not a good idea, but there are situations where it might be useful for your human soldiers to flee a battlefield and let a commander know when a situation has changed dramatically not in their favor.
Take, for example, sending some troops across a mountain range, over a plains biome, and across a bridge to try and attack a force on the opposite side (i.e. a city territory, a set military barracks, etc.).
The other side might setup a few troops on their side of the bridge as scouts, and when the Undead start to cross it, allow them to, in part - and then destroy the bridge.
The Undead on the other side of the bridge might then try to walk across or use undead bodies as a bridge to cross over, but they could be susceptible to Greek fire in the river between the bridge, or just being pelted with arrows as they attempt to do that.
While a set of scouts try to also take on the undead that have successfully crossed the bridge, or just let the much smaller force try to take on the barracks or city. If they allow 10 Undead at a time to try and attack the barracks, it doesn't matter if you send 100 or 10000 Undead soldiers - 10 Undead at a time is presumably manageable for the opposing army, barring them automatically resurrecting on the site as they die.
If this was a human force of soldiers in the same situation, the soldiers crossing the bridge before it was destroyed may respond to the destroyed bridge by surrendering, or fleeing to the mountains, while the side that hasn't yet crossed the bridge may provide covering fire for them or begin fleeing themselves, perhaps back across the path to the commander to report on what happened. They may also setup camp outside of the river area while sending a smaller scouting party back to ask for reinforcements or bridge building capabilities, or additional orders. If they encountered the Greek fire and had no existing knowledge of it before, they would also want to relay that back.
Similarly, if they encountered Greek fire while on ships, humans would understand that not only are their lives important to get back, but so are the ships that were built - especially if it was followed up by catapults throwing rocks at them to try and sink them. Some orders are given with a bad understanding of the situation.
Additionally, you can give a human force - especially a human commander - enough information about their task that they might catch on to information that indicates that the information higher command has is incorrect, or a possible trap, and update them as necessary. 164
For example, if you send them to take on a large group of the enemy army force to keep them away from a castle you're planning to siege, and they show up and find only 90 soldiers holding up the outpost instead of the 900 you thought were going to be there, a human commander might fight the 90 soldiers quickly, but also send a messenger back to alert their high commanders that their information is presumably out of date; there's a discrepancy in the force they were told to attack and the force that was there. Useful information if you have a mole in your team providing false information.
Finally, humans will likely retreat if they feel that the enemy army is starting to encircle them, or attempt to force them into a worse situation. The Undead may still want to fight the enemy force, because that was the order they were given, and they may not have gone to war with the enemy force if they weren't compelled to anyways.
Furthermore, the Undead have no inclination to prevent the enemy forces from killing them and resurrecting them against you.
If your Undead fail to successfully defeat the enemy and instead get themselves routed, then now the enemy has a full group of soldiers they can resurrect and send back against you. As a bonus, they're already equipped with all the equipment they really need.
You did equip the Undead with armor and weapons, right?
Your human soldiers, upon realizing they may be in a losing fight, and that they may be resurrected as Undead, and may deliberately either flee to make it harder for them to be resurrected, or get themselves stuck under crushing buildings or rocks, or deliberately try and break and destroy their lances, swords, and bows, such that they can't be used against their allies. At the minimum, they can start denting their weapons on purpose. Not ideal, but they can try and avoid giving an advantage to the enemy - maybe they cut off their sword arm just before dying, or their legs, or both legs and arms in a final act of defiance. 164