Among other things, that's what makes the default Perisno faction surprisingly good.
Darkforest Cavalry (maxed commoner recruits) give you a huge pathfinding bonus. Alongside the speed bonus for being cavalry units, a party of your companions + 50 Darkforest Cavalry moves VERY fast. And Great Knights let you train up full armies much faster, so while a full army of Perisno units isn't as scary as a full army of Elves, the Elves are much less expendable. Exploit this mercilessly.
AI Lords often have common units (and sometimes special ones!) that give them stat boosts as well, so don't be afraid to use your own! For example, I'm pretty sure that the Tolranian Count Rafaracus can move so damn fast because of the Tolranian Yeomen in his party. He's the reason why I had so much trouble at the start of the game trying to raid Tolranian villages, because his party is unnaturally fast for a non-cavalry party. EDITED: Turns out AI Lords just have support stats of their own already, thanks Leo.
For another Tolranian example, the Kingsmen seem to apply a huge leadership bonus, and the Galwe Royal Engineers give you a huge Engineering bonus so they're essential for building up your towns quickly and laying siege to Siege Tower castles as quickly as you can.
These bonuses seem to be cumulative and apply directly to you. For example, I have 6 Elintoran Master Scouts, and I have 10+4 Pathfinding... but I never put any points in Pathfinding, and my Attributes are 30 STR, 30 AGI, 16 INT and 28 CHA. Those points in Pathfinding are all from special units giving me bonuses. That's how good they are. EDITED: Turns out they're giving me the +3 bonus, but I have other Pathfinder units adding to that total apparently.
To protect special units that can't be easily replaced and are bad at combat, I have "Support Units" assigned to my 5 key, and I place them right at the bottom of my party, and tell them to retreat in field battles. Darkforest Cavalry are easy to train and decent fighters so I don't put them in this category, but units that I DO put in this category are otherwise weak units with incredible skill utility, such as;
- Elintoran Master Scouts (Tracking 7, Pathfinding 7, Spotting 7)
- Galwe Royal Engineers (Engineer 8! [decent crossbowmen, but too valuable to lose. Also have Tactics 10, bonus doesn't apply though.])
- Draharan Herbalists (Wound Treatment 6, Surgery 6, First Aid 6 [not actually weak but too valuable to lose])
- Gehorsod Helend (Wound Treatment 8, Surgery 8, First Aid 8 [very strong Falconian horse archer, but insane medic capability. Must protect at all costs])
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The point of all this, is that troops are now more than just a combination of their Weapons Proficiency, Attributes, combat skills and weapons/armour. If any unit has 6 or more in a support skill, that's something to take close note of.
Leadership units such as Tolranian Kingsmen, Geldarin Dukes and Perisno Champions Perisno Holy Champions (among many others) will form a key part of keeping increasingly large armies together. When you become a King and get the party size bonus, you'll still be limited by morale! Leadership troops alleviate this problem and provide considerable morale bonuses to the whole party.
Trainer units such as the Perisno Great Knight, Reich Dragonritters, Falcon Renweards and Hakkon Imperial Knights also help you level up your party much faster, so if you woke up in a cold sweat at night muttering "I don't have enough Crimson Rangers yet" in your delirious stupor, you can use these units to help you train up full armies in a speedy fashion.