I concur with IMP with regards to auto cannons. (which is general term for cannons with high rates of fire).
As for Gunships in general I think they work fine but you really must be up against an enemy with poor to no AAA or SAM capability or have killed those assets. I've used these on occasion and even though I didn't kill but one unit in the target area, the others were suppressed to the point it was a mop up operation only. Another point with Gunships is targeting, they will mostly kill everything in the designated target, just make sure the target hex you give it has a target, as Gunship will show up and move down trees and weeds exactly were you told them to fire.. These guys should be pricey as this is a very specialized kind of strike and certainly not for every battle situation.
My best Gunship strike was Puff the Magic Dragoon against troops in an area I wanted the put Marines ashore in their LTVP's and a few landing craft. I had two Puffs’ on call and had them hit two targets 150m away from each other and 50m inland. Once they finished with the area I had a landing area 500m wide with nothing but broken troops holding it, I lost not a single Marine and only one Puff drew fire leaving the scene. Pretty good value for the buck I thought.
How I have lost a few too, most because I wrongly assumed the EN AA capability! A slow circling target for a SA7 is easy meat!!
Bob out