No sass here, nor any disrespect, but either your mis-reading or we have different books.
The only limits on Special units for us is on Warmachines, ie: Cannons, Grudge Throwers and Bolt Throwers.
Again no sass or disrespect. I also suggest you read the list entry again. They have 3 main ways to attack, Impact hits on a charge, a single Divebomb attack that can backfire (it misfires, no mention of the miscast table), and a choice of 1 of 3 types of guns that can change the role they play. Steam guns are iconic, they are a weaker template attack, Brimstone Gun hit hard and punch through armour and can be fired while moving and if charged, Clatterguns sit between the two as far as damage goes, but can be fired after a March move.
They give us much needed harassment power, a way to silence Warmachines without committing our own to the task, a way to clear screens and skirmishers away from the battle line and most importantly show the Dawi are the great engineers and builders that they are portrayed as.
If I wanted that kind of power to add to my list and not take Gyrocopters, then I'd be pulling in an Allied Contingent of Calvary. Which isn't a bad option, I have a detachment of Knights of the White Wolf that fit perfectly alongside the rest of my units.
It'd be like cutting off Slayers because they don't wear armour, Irondrakes because they are walking cannons, or Rangers because they are sneaky.
Now all that said and done, Gyrocopters are good. Thier biggest weakness is that they pull from the same resources we use to get gems like Hammerers and Cannons and the like. Pull too deep and you might lack the Steel to carry on the fight.
That said, groups of Gyrocopters have a hidden strenght and weakness. That being Unit Strength. A pair of Gyrocopters gives you a unit strength of 6, which lets you capture objectives but now causes panic in your army when destroyed, losing a sole Gyrocopter is no cause for concern, but a flight is worrisome.
There's also items that can cripple them, like the Falcon-horn of Fredemund, which turns of Flight for a turn should they pass a Leadership test(7-9). It's also not a bound spell so we can't dispel the effect, and the can use it every single turn past the first. 10 of the last 10 games I've spent fighting Bretonians and I've learned that I'm only guaranteed a single move, so I'd best take Clatterguns and get them into position to do some damage before they start limping along with a 2" march for the rest of the game.