I'd say it's one of the easiest guns to use. BOOM, BOOM... enemy balloon is down in two seconds and there's nothing they can do to stop it. Now that they're falling, I can get into a better position and have my gun reloaded way before their balloon comes back up..
Balloon repaired by xxxx
Balloon destroyed by yyyy
One engineer is not enough to repair the balloon before the Carronade is reloaded and having two engineers on the balloon leaves the rest of the ship vulnerable. Once the carronade is in range, your balloon will be popped, guaranteed, over and over. Having an allied ship (that isn't busy getting attacked themselves) rescue you is a best case scenario. There aren't enough guns that can aim high enough that will actually bother an attacking blenderfish. A Drogue Chute isn't a counter, it just prolongs the fall. A gunner using Loch rounds can easily kill any ship alone. There aren't many weapons that can disable and kill at ranges beyond a Carronade; only the LJ, Merc and Flak come to mind, and those are significantly harder to use, not to mention you'd have to be camping in order to not be snuck up on. Once your balloon is down at max range of the carronade, that ship can close within arming time before the balloon is back up. Trying to kill them before they get too close is the best option, but as soon as they're in range, you've lost an engineer to repairing your balloon while they still have a full crew.
We don't all get to fly with level 8+ crews who know what they're doing. Can it be countered? Yes? Can it be countered by crew that hasn't been informed of EXACTLY what they have to do? Not likely. Just because the high level clan can survive it doesn't mean the newbies and random crew ships can.
It's OP because the GENERAL POPULATION of players on GoIO can't counter it.