Some people say Marksdwarfs are broken, but I find them the easiest to arrange. (Maybe avoid the 'broken' stuff.)
My current fortress has just one marksdwarf unit and further (fairly full) units dedicated individually to spears, axes, maces, swords, and whatever else I have, plus a couple of other units for basic training of more recent immigrants (with worthwhile military skills, but not yet put into gaps in other units or used to create new ones), plus a number of immigrants that I've just not gotten around to getting even to that stage. With nearly 200 dwarfs in, I impose a schedule that's either a third-on, two-thirds off or two-thirds on and one-third off, so that they get civilian lives. This includes the marksdwarfs.
But due to the design of my fort, I very often call upon the marksdwarfs to deal with most foes (actual enemies or troublesome zombiebeasts that are frightening the civilian workforce). That's the one unit. And they deal with them all quite well.
Nearly all. The Brush titan made of amber was injurable but seemed unkillable by bolts (generally the gamut of wood, bone and silver... don't think I had any steel ones), and when I finally got fed up with it I just sent the marksdwarf unit in to bite and bash it to death.
Also, on a recent occasion, I'd only just released the marksdwarf unit from killing off some pesky zombie livestock when some more zombie livestock popped onto my territory, so I sent the next unit along (the speardwarfs). They tended to get bad thoughts about encountering undead animals, but they dealt with them Ok (having spent most of their 1/3 or 2/3 time training, whichever way around it was with them). The marksdwarfs at least don't get that.
I just don't like not making use of a significant skill (insignificant ones might be all the fishery ones, the way I play forts, all military skills, including teacher and student and observaton, I consider to be significant). To the end that while I won't intentionally make any more than two miners, anyone who arrives on the map with significant expertise in mining gets allowed to mine, meaning that out of the nearly 200 total, I have five true miners, all now legendary, and it's only the dabbling ones that didn't. And two of those newer miners came with some other military skills, so they've been doing both mining and their combat training part-time, since they arrived. (I know there's a military equipment issue with miners/woodcutters, but I just like the idea that at least a couple of my miners might surprise anyone they accidentally come across, down in the depths, and naturally they're armoured.)
So, anyway, if someone comes along with any skill in mace, I plan to get him into the mace-squad. And I've equipped them with maces, and they've been training a lot. So if I ever need them, I can use them. Other than that, right now I would be happy to field an army totally of marksdwarfs (current lot can hold the line while the others get trained up, or just zerg-rush them, with or without zerg-vollies being fired) and I'd be happy.
And my marksdwarfs are metal-clad and shield-bearing. Not all steel. Neither my map nor my home civilisation has any sign of iron ore, but I have imported steel bars and steel items I can melt down, so in six years of operation I've managed to make some equipment in that, rather than copper... and the local silver that I have used for the hammers/etc. But they seem happy enough with it, and have come up against marksgoblins and triumphed (across an impassible ditch, but without the advantage of fortifications on their own side, and in a roughly 1:1 ratio, where I'd normally be looking to have bent the odds much more in my favour.), so they're far from vulnerable to being range-attacked in their turn.
Did that answer any questions? I think it did, but I've probably drowned it in other irrelevant stuff.