Any simple roll of dice in combat would provide plenty of randomness.

Let me give an example of why I get frustrated by being ignorant: In the good old 1995 days there was an excellent turn based strategy game called Warlords 3, wherein your units would have a "str"-rating of x (between 1 and 9). They could move x spots per turn and take x (1-4) hits before dying in a fight. Now the combat system was that a dice was rolled, the unit's str was added to the result of the dice and the loser took a hit. The dice being rolled was the randomness - a dragon str 9 hp 3 could get flattened by a simple group of light infantry str 2 hp 2 if they were lucky enough with the dice rolled.

However, as it always was with such things, the dice being rolled could be anything you wanted in the particular game - from... I think it was a 8-sided dice to a 50-sided if you like a bit of randomness to spice things up. Naturally this meant that in campaigns the dice would vary in size depending on the map. Thus, you might wish to improve a certain unit's str to increase their chance of winning massively in a 8-sided dice-map, but you would much rather add more hp or more movement to a different unit to increase its chance of winning in a 50-sided dice game (as its almost a 50/50 who takes the hit, a hit point is about 25 times better than a str point in such a case on a mobile unit working alone, as a surviving unit gets all its hit points restored).

The thing here is that while its random, there is logic behind the madness. I cannot control whether my units do well or not, but I can understand the randomness and how it works - all the mechanics are clear. Its like playing poker to a certain extent. Sure I don't know what cards you have, but I know you have a combination of the 52 in the deck.

Not knowing what charge bonus does (does it affect 1 attack? For 3 seconds? What if you just run into a unit and don't specifically order a charge) or how attackskill works vs defense+armor (why do Katanas beat Naginata Samurai when the Naginatas have more attack+defense+armor - is it attack being more effective, defense or armor being less effective or what gives?) just means Im drunkenly stumbling around in the dark as to what Im doing. If I think attack+defense+armor equals chance to win a direct melee fight, but your Katanas humiliate my Naginata, it feels like you just pulled out a joker and beat my full house with a "lolapallozza"-combination that works in this particular casino. I wouldnt know what just happened or why it happens consistently (aka not as a function of randomness). Such things bother me :p

But until someone finds out how these things work I will settle for improving armor on anything that cant shoot. Except maybe Yari Cavalry... that depends whether or not charge is any good, but since I dont know I guess giving them more +attack would make them significantly better against enemy samurai archers and katanas that wont easily break and rout due to morale shocks.