I suspect the difficulties with military come from two sources:
1)Confusion from the new control system
2)Extremely strong enemies
It seems to me, that the difficulty with the stronger enemies is the difficulty to actually manage landing a hit. I just destroyed my 3rd or 4th goblin ambush in this fort. In that ambush I fought ~20 goblins with 20 of my military dwarves. If they had been normal goblins, I would have killed them without a single death from my side. The problem was the goblin swordmaster that arrived with the ambush forces.
The swordmaster held off 5 of my dwarves without getting a single injury. Note, none of my dwarves were "<weapons> lords". I had to assign 20 of my military dwarves to kill the swordmaster, and then the goblin finally got killed. The Swordmaster died when my axedwarf hit the throat with a silver battle axe.
My point here is, putting unexperienced dwarves against "Master"-level enemies is a suicide, unless you can throw an entire army against it/them. Hell, even "avarage" dwarves have trouble with "Master/megabeast"-level enemies. You need
squads of legendary dwarves with full metal gear to counter the "megabeast"-class enemies, it seems.
And I like that fact, personally.
Right, I think I've figured out what's going wrong here. I've save-scummed twice just before a forgotten beast spawns. Before it spawns, my military will happily follow any and all kill orders, but when a forgotten beast appears and I set my squad to attack it, they instead completely ignore the kill order.
I'm going to see if they'll at least follow a move order.
I got that effect with Giant Eagles with my novice military recruits (was first days of my fortress), when I ordered them to kill a thief. It's almost like the dwarves have a "Holy shit holy shit that thing is HUGE holy shit it's gonna kill us" -effect when a creature much bigger/stronger comes.