As a warrior who has fought thousands and thousands of battles (mostly with raging barbs) here are five of my observations on attacking:
1. I think the Combat Odds are out of whack. I have lost way too many battles I was favored to win in the 90+% range. Plenty of them in the 99.X% amount and one, only one, memorable time I lost a battle when the odds were 100%!!!
2. I think the Combat Odds for 80% or lower should be saying those are the odds you will LOSE not win. That has really been my experience, and I will not chance an important unit in a battle 80% or lower.
3. Defending odds are also sometimes out the window I have found. For example, yesterday I was defending a key resource (copper mine) with a maxed out Warrior who had fortified the position to 25%. He even had 40% against mounted units. Along comes a lone Worg Rider and takes him out, has a movement point left and destroys the mine! Fortunately, I did a quick save (c'mon we all do this for key battles

) and decided instead to send my archer there - again maxed out on XP. He was killed too. Finally, I put a maxed out Hunter there, and, you guessed it, he was also dispatched by the Rider. Nothing to do but give up the resource and quickly kill the offending unit. But, the point is I have to think the AI's combat odds against any of those units was really, really low - yet he still won.
4. I agree with Kael that it should be more like a chess match where you set up the proper offense/defense and the right units with the appropriate attack/defend value. To then lose when the odds are overwhelmingly in your favor is frustrating. But, hey, I guess it history it has happened over and over again. I am just saying in this game, if you have a 99.9% combat odds of winning, you should win 999 out 1,000 battles. I don't think that is necessarily the case.
5. Lastly, I think the terrain features can be murder on the combat odds. Trying to kill a unit on forest/hills with another until is very tough. Maybe that is as it should be, but it sure can skew the combat odds.
And...I am still trying to find out a way to get those nasty barb Lizard Men early in a game who park themselves in the water waiting to pounce. They have perhaps the best terrain advantage of all!