Irondrake seems terrible, the three unit that do look good in the duardin side are Runelords, Ironbreaker and Hammerers.
I don't think a mono race build could be efficient in City overall, but well.
I would recommend multiple Runelord, if you are not gonna cast magic, at least try to deny some.
20 Iron breaker with Order (Hold the line for 4+++ and -1 to hit) and Prayer (Oath and Steel for -1 to wound +1 save) buff is realyl really tough.
Hammerers are a decent hammer, especially with the Strike Them Down order. They pair ok with a Warden King if you want to take one, even tho a runelord would still be better I'd say.
I don't like gyro's, they're way way to expensive, so much so that when I saw the point cost, thought that it was units of 2. BUT, if you're going mono duardin, then you'll need them, because you have nothing to score otherwise, with how slow the rest of the rooster is.
I guess a single unit of Irondrake can be "justifiable" just to use the Master of Ballistic Command trait, but they're very underwhelming compare to every other shooty option in the CoS index