Piraeus: a double edged sword
The Piraeus has two effects: you get one papyrus or glass every turn, and you gain an extra turn when you build it. As we explain in this post extra turns are very powerful in 7Wonders Duel. Since papyrus and glass are rather rare resources, getting one of them every turn is also powerful.
The reason that the Piraeus is not completely overpowered however, is that the resources effect and the extra turn effect of the Piraeus are hard to combine. Because to obtain the maximal benefit from the resource bonus, you want to play the Piraeus as early on in the game as possible. This way, you get the most use out of the free resource. But, extra turns generally get more powerful as the game goes on. Because extra turns can be leverages to obtain specific cards, and the cards that are available in the later eras are much more powerful than those in the early eras.