Have you noticed the quern takes a long time to complete a job? Is it better than a millstone? Why would you build a millstone instead?
I did a fun test. 7 dwarves, nothing but tools, bags, and food. They cut trees, mined stone, and made 1 quern, 2 millstones, and a mechanic's workshop. Then, I started one dwarf on milling some plants at the quern. The other 6 started working on 3 mechanisms, and building the walls and floors to support a windmill. Once they got all of that done, they built a windmill, a gear assembly, and two millstones. Then, I started a dwarf on milling plants in both millstones.
The dwarves at the millstones beat the dwarf at the quern, who was still milling the same plant he'd been milling all this time.
The downside to a millstone setup is that it requires mechanical power, and that involves making either a waterwheel or a windmill. This is a downside for two reasons:
- this takes more materials (though, while the difference between these and a quern's cost of 1 boulder is large, the actual material cost is pretty ignorable).
- this creates potential entrance points for invaders, who can fly up to your windmill and fly down through the axle or gear assembly, or swim up through where your water wheel is located to access the inside of your fort. Neither the windmill nor waterwheel block movement through the machines, and the axle, gear assembly, and millstones are likewise easy to walk through.
Want to be more secure? It's possible. If your flowing water source is a brook, then as long as you refrain from breaking the surface anywhere except under your water wheel, you can easily wall off the water wheel and millstone setup with underground access and count it as secure from anything except that which is already in the brook's water. The wiki's
water wheel page details ways to make a wheel setup underground, secure from invaders. Windmills are sometimes easier to set up, unfortunately they stop working if they lose access to the sky (they do still work with walls all around them, but that doesn't protect you from fliers). The best you can do to secure a windmill is internal security in the underground passages that allow your dwarves to access the mill, or to access the mill only on the surface, accepting that during a siege it will inaccessible and likely destroyed.
The security concern just doesn't exist your first year, and in the first month you can easily have a mill setup built and providing your fort with valuable grain and dye. Then, as you have a more robust fort below ground to protect, you can start working on making sure your mill is not an access point for invaders.
Or, you can build 7 querns, and all of your dwarves can spend their whole first year milling fewer than 30 plants. Maybe you can already tell that I have a low opinion of querns.