Hi Maloncanth.

The trick is to read the 'terrain-type' information on the province parchment (before you invade it).

Terrain Types
Lush
Steppe
Temperate
Arid
Rock/Sand Desert


I think the desert terrains are a definite but I'm not aware if fatigue factor begins to kick in at Arid, or in Temperate or if, in fact, there's a steady progression right through the entire range.

Increasing levels of armour count as "encumberance" so, even in Lush terrain, Silver/Gold armoured troops will tire at a faster rate than their lesser compatriots. They can certainly hold their own in a protracted slugging-match but don't let them run around too much - get lighter-equipped units to do that. In a way, it's a good thing that different provinces you own will be at different levels of progress, with armourers.

Also bear in mind that some unit types are classed as 'armoured', even in their basic, un-upgraded form, so they will reach the threshold sooner than other types. So, for example, FMAA are more heavily penalised by armour upgrades than peasant-class units, like Gallowglasses/Clansmen/Spears/Militia.

I'll leave others to expand on the 'desert armour limit' part in more detail. I recently had to use bronze-armour (+2) FMAA in the desert and they really suffered badly (mobility as well as casualties). Rather than demolish a perfectly good armourer in the province they came from, I'm getting another province (Navarre w. Metalsmith) teched up to make them but with no armourer at all.