Well, In FoE I've run into a "power curve". Ambushes that can butcher a small caravan and seriously threaten a militia of 5 with skills around 5, don't even make for a good live combat training later on. Whole squads of enemies who can quickly kill any caravan, can be killed with 2 expert military. The problem is staying on top of this power curve and having enough well-trained and equipped soldiers to stand up to the threats that show up. I screwed this up several times and needed a few years of lock-down to train new soldiers. This is why I think you need to throw about 10 soldiers with skills of 10+ and good steel equipment against sieges of Changelings and do it several times to draw conclusions as to how good they really are at fighting. But then, I'm not that good at DF military.
I want to share an anecdote about CMs. In FoE I got a pony wit a dagger CM, so I drafted him. He got full security armour, a dagger and 4 shields (Unicorns in FoE have 5 graspers and a dagger only takes up one). Steel Ranger ambush arrived, so I ordered everypony inside and went on a lockdown. The daggerpony stayed outside, so I thought I lost him. But then, amazingly, he stood up to their leader, while the rest of the Rangers butchered a caravan. None of the duelists was able to overpower the other's shield wall and my pony got to legendary shield used and knife user, until the rest of the Rangers came back after they were done with the caravan and helped their boss. I was even tempted to open the gates and throw the rest of my military at them, to see what will happen.
This is an anecdote about CMs, because the daggerpony had about 250% learning rate for knife user, which is why he was able to rise to Legendary so fast (and with very little attribute gains, according to DT 20.0). His shields were so good, because they were of decent quality and they stack, that is if the attacker bypasses one shield, he still has to bypass the other three to hit the target.