Certain faction-unique units.

[FactionHeir]

Thats not quite correct.
Voulge Militia start with armor level 2, upgradable to silver 4.
Halberd militia start with armor level 0, upgradable to gold 3.
Voulge: 5/6
Halberd: 5/1
Post upgrade:
Voulge: 5/8
Halberd: 5/7

So the difference in armour is really 1 point and they are otherwise exactly the same. Note that also Voulge Militia are 90 florins more expensive per unit, so overall they are equal in cost-benefit.

Voulgiers are 7/8, upgradable to 7/10. Cavalry can charge them head-on with minimal losses while breaking their formation and massacring them due to their low defense. Besides, they are very slow and exhaust quickly as I noted earlier, so not a good choice of unit if you can replace them with footknights (which are only 60 more expensive and have 75 more upkeep but more than double their stats).

[Robespierre]

alas! halberds ain't what they used to be. its as my old man says, you just can't get the halberdiers nowadays. i yearn for the days of mtw, the days of yore, when they were to an attacker what a large helping of xmas pudding is to my intestines. alackaday, alackaday! that this should ever be. there is no choice for a devoted halberd lieutenant but to play the danes.

[marrow]

It truly puzzles me how the whole concept could have gone down the pipe like that. Started off with Naginata in STW, which were an awesome holding unit, continued as halberdiers in MTW who ,again, although painfully slow could hold any gate/bridge under hail of arrows. And look at them now! Bloody shame... halberd militia, voulgiers, billmen - they all suck! I'm a big cavalry fan, but even as such I must admit there is something deeply troubling about being able to rip through a double line of billmen with a bodyguard unit. Bloody shame...