Well, when you look at them carefully, Foehn have all sorts of cracks to exploit.
> Got a Harbinger timer ticking down? Take advantage of Foehn's poor spy counters and infiltrate their Harbinger Tower.
> The Boidmachine is just a tad inaccurate. I've seen shots outright leave the map on several occasions, much to my annoyance/amusement (depending on which side the thing is on).
> Foehn infantry are strong, but those exosuits make them weak to Magnetrons or EMP. And while Foehn infantry are immune to dogs/spooks, they aren't immune to Terror Drones or Chitzkoi. Only 2 infantry units have immunity to them; Uragan (because he's airborne), and the Synchronaut (presumably because the TD/Chitzkoi can't find the bugger).
> The Quetzal's got power, but unless you've got multiple Coronia factions on the same side that shield support power only lasts 20 seconds, and then they're much more vulnerable... and they can neither promote or provide money through the reprocessor. And they have no real defence against air units, and every faction has access to some form of air to air firepower (Rocketeer, Thor, Wolfhound, Vulture, Gyrocopter, Invader, Irkalla, Vulture).
> It also takes Foehn longer to tech up given they have to build more structures to hit T3, and are slowed down by the money issues they tend to have in the early game.
> And the aforementioned lack of spy counters makes it easier to infiltrate Foehn barracks/war factories and pick up veteran bonuses, levelling the playing field a bit.
> Unlike Spies/Saboteurs/Infiltrators, you can usually see the Clairvoyants coming, making it easier to prevent them from getting the same bonuses/shutting off your power/stealing your money etc.
It's not necessarily so much that Foehn are straight up OP as you need to look for their weaknesses and adapt your strategies, rather than just trying the same old stuff that works against most of the other factions. You can't necessarily meet them in a straight up fight tank to tank/infantry to infantry fight, so you need to find ways outmanoeuvre them.
So far in my anti-Foehn skirmishes, I've only really had trouble holding them off as Epsilon, and then only until I start spamming sufficient MBTs and gatling tanks. After that, they're pretty much just as exploitable as other factions.
...Although I'll admit there are still balance issues, such as the sky high Reprocessor bounties.