Armour question...

[King Flambard I]

As far as I believe, no. You are equally suspectable to crossbow bolts in any type of armour as the bolts are just amour piercing. I'm not sure how a unit can have equal defence rating in chain mail opposed to full plate though. Generally, full plate will offer a lot more defence than chain maail, granted it was just an example.

As for any other impact, i'm not too sure at all, again, I don't think so. It would make sense for movement speeds to be restricted in full plate opposed to padded leater for example but I don't think that this is noted in game. At least I haven't experienced this.

[Thurak]

I'm not sure how a unit can have equal defence rating in chain mail opposed to full plate though.

The base defence is the sum of three values: Armour, Shield and Skill. So it is easily possible for units with (very) different types of armour to have the same defence value.

[Old Geezer]

This is why armor piercing missiles to the rear of a unit are so devastating. The shield doesn't count nor does the skill of the defender.