ePilgrim, the statistical analysis is fine and I don't think there are many people who will classify the assault re-roll as unbalanced. The question is does it seem 'stubborn'? All the analyses in the world can't quantify the feel of the rule.
While the rule works fine, I do actually think there are better ideas out there.
I don't like the saving for hackdown kills because it is not really representative of being stubborn either. Breaking is breaking; it is a loss of organization. Hackdowns represent being routed (often confused with breaking). When a formation is brought to this stage it doesn't matter how stubborn they are - they will flee or be cut down as they turn their backs to the enemy and RUN.
Stubborn IMO is a soldier who stands boldly in the face of adversity, regardless of whether it is smart or not. It is tied to morale which can be defined as either logical or emotional or both. Since Blast Markers are the only real measurable characteristic of morale in Epic: Armageddon, it stands to reason the stubborn rule should be tied to BM's only.
Ex. Ayone who has seen The Return of the King movie will see examples of stubbornness. As the Orcs are being bombarded by trebuchet launched chunks of granite, the Orc commander tells his men to stand their ground. At the very last moment he takes a half step to the side as the granite hits the ground and then spits at it. STUBBORN.
Of course, later on the Orcs are being charged by cavalry and they stand their ground resolute (also stubborn). But as they all realize that they are about to be killed, the Orcs not only break but are routed, turning their backs and running as the riders of Rohan trample and spear them to death. Being stubborn did not help them here.
ePilgrim, I know you have done a detailed battery of tests, but from our discussions off line you know that there are combinations of ideas that have never been tried. You know my ideas so I won't regurgitate it again.
But to drive a point home, this isn't an attempt to beat down on your idea. If you decide to leave the rule as-is, I think your list will be fine. It works, it seems balanced. It also feels wrong and playtesting will change nothing.
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author of Syncing Forward and other stories...It's a dog-eat-dog world, and I've got my Milkbone underwear on.