One problem imo - having a helmet with 100% coverage means NPCs will stand and bounce weapons off unconscious peoples helmets 100% of the time for ingame months without killing them..
Bladed weapons, yes, unless there is enough force behind the blade to bruise through the helmet then the victim is completely safe. Some blunt weapons will also bounce without enough force, but persistence is rewarding.
As with real-life close combat my mod actually puts a lot of weight on to wrestling, and the tactic of occasionally manipulating your opponents armour (in this case, removing the helmet) to exploit their weaknesses. I realise the AI cannot normally do this, and I compensate by balancing for common sense; sword vs hammer in armoured combat, the hammer is going to (usually) win, period. Otherwise the sword has the advantage in unarmoured combat.
So I found something inconvenient when I tried to play Fortress Mode with your mod. Your pickaxes are much larger than vanilla picks, so they cost 324 embark points. I think I'll stick to Adventure Mode with this, because that's probably what you had in mind anyway.
Ah... I hadn't thought of embark points beyond adding the hatchet for a cheap woodcutting axe to start with, though that could also be false now too. It's been a while since I tested Fortress mode, and you're right about Adventure mode being my main concern. I will look into fixing the pickaxe problem, scold me for not thoroughly inspecting Fortress mode!