If pyramidions were overpowered, competitive matches would consist of four pyramidions. Clearly that's not the case. Pyramidions are easy to fly and crew on, which is why they seem overpowered for many new players. As a pilot when I don't feel like doing as much work, I'll fly a pyramidion. The separation of hull/engines and balloon/guns allow for most beginner crews to be effective (once they understand this separation), plus forward guns are easy to aim.
That being said, it's certainly not overpowered and honestly I'm not sure if I can explain it any further than some of the posts here already have. It has a large, easy to hit hull, and doesn't maneuver easily. If you accidentally miss your target by overextending (which newer pilots often do, when they don't have a good grasp on how each ship moves), forget about it. Your enemy will easily take advantage of your massive blind spot and you'll be down in seconds. Easy to learn and a sturdy brawler yes, but easily countered once you realize other strategies exist.
And the argument that "just because it's possible to counter it doesn't mean it's overpowered" is silly. Everything will seem overpowered in this game if you don't know how to counter it. If I don't avoid a galleon's broadside, the galleon will seem like the most overpowered ship in the game. The same principle applies to the pyramidion. If it wasn't able to be countered, I'd agree that it's overpowered. But that's definitely not the case.