If we compare anti-land and anti-air turrets and their maximum ranges (i.e. excluding a secondary, close-range turret like the Desolator's), we'll see something like this:
Anti-LAND
Metal ------ Turret Name --------- Range ----------- DPS
90 Lotus 460 72
100 Picket 610 25 (reloads slowly; deals 312 in burst)
220 Stardust 410 450 (with Splash)
250 Faraday 460 0 (1200 EMP)
400 Gauss 560 (terra helps) 100 (bunkers down when not firing, penetrates)
420 Stinger 620 189 (850 damage in a single burst)
1600 Desolator 650 400 (Requires power, 1201/projectile, large splash)
2200 Lucifer 1200 400 (requires power)
2500 Cerberus 1850 180 (I think the wiki page is off on reload time - it seems to reload faster than once every 10 seconds)
Anti-AIR
Metal ------ Turret Name --------- Range ----------- DPS
220 Hacksaw 480 80 (deals 1200 in a single burst; long reload time)
280 Razor 1040 149 (bunkers down when not firing; often misses)
450 Thresher 1000 264 (Has splash)
900 Chainsaw 1800 225 (225/shot, homing, one shot per second, HUGE RANGE)
2400 Artemis 2400 890 (1602/missile, requires construction of missiles beforehand)
When you look at those numbers, you'll notice that AA defenses hover at about 1000 in range, which is 1.6x the max range of most ground-based defenses. That's probably fine, since you want to get your shots in against fast-moving planes before they deliver their payload. But then there's the chainsaw - 1800 range is HUGE, and on a purely DPS level, it outdamages the STINGER (Yes, Stinger deals more-per-shot, but when an anti-heavy turret can't keep up with your long-range turret, something's wrong). With that, a chainsaw can target (and almost always hit!) targets that are in Cerberus range. It's anti-air artillery, which makes it impossible to keep flying constructors anywhere near the front lines, and makes it hard for a player to set up a sizeable airborne attack. Chainsaws are oppressive - they make players switch off of air factories too easily, and completely negate air-based strategies (like transporting units) in the mid-to-late game. Air players can't participate in building distant bases to take advantage of their mobility either, because their constructors are too-easily shot down. (Krows don't die to Hacksaws - players don't even BUILD hacksaws! - they're dying to the incredible range and overlap of current anti-air. Once you commit to a Krow attack, you literally have nowhere left to hide.)
Air planes and gunships are both factories that are not newbie friendly I think that a major factor in this is that anti-air defenses (like the chainsaw) have too great of a range. It's rather trivial, both in terms of resources spent and player skill to execute, to build a chainsaw and completely lock down half of the map. Moreover, if you can build a chainsaw somewhere even close to the enemy's base, you can shoot down their constructors and camp their factory quite easily. Suddenly, every single unit they've built previously is useless, because they can't stage an attack on the chainsaw when it doesn't leave room for a staging ground. This is also why you never see transport-based strategies except for cheese.
I see a few solutions to the problem:
-Make Chainsaw a high-damage, medium-range Hacksaw that doesn't have to reload. It would certainly fit the model better (those missiles on the model are HUGE). It would still have relatively low health and make it worth taking out, but it wouldn't lock down the enemy anywhere near as often due to range)
-Make Chainsaw cost more. If it's a 1500 metal investment to lock down that large of an area, then it's worth it to punish them for building it.
-Require an energy grid. This makes it harder to deploy offensively (where it's the most problematic).
Anti-air defense does have to be effective. It has to take down units of two very different classes: fast-moving airplanes and slow-moving gunships. Fast-moving airplanes are best taken down by high-damage bursts, though this is mitigated by the swarms of swifts available to the air player (requiring skill! yay!). Slow-moving gunships, on the other hand, get shredded by long-range damage over time, since it takes so much longer to retreat. Right now, the current turrets are very strongly in favor of shredding gunships (though it's still oppressive to airplanes).
We've already got a turret that's fortified and does well against slow-movers (Razor), an medium-range splash damaging turret that hits everything (again good against gunships due to speed vs. range), and a burst damage turret (better against air - you might take out one gunship/turret, but gunships stick around). Adding another turret that deals higher damage to quick attacks would go pretty far, so I recommend the first option, above.
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An alternative suggestion: move Krow over to striders and give gunships something resembling a stealth-generating gunship (or enable transports to carry a stealth generator and have it work) Oppressive ranges don't matter if your gunships can stay hidden. Stealth revenants, caused by irises on the ground, can be powerful, but right now are limited to defense since the iris can't climb surfaces like revs can. It also requires building and morphing a cornea or facswitching to cloakies. It's supposed to be a heavy raider, right? Let it raid!
Planes are still hurt by chainsaw ranges too, so the suggestion above doesn't fix the issue for planes...