I've played as the spanish several times, and they don't seem overpowered. However, I've heard people complain about it. Could someone tell me why they're overpowered?

They are not overpowered anymore. They used to be quite some time ago in old ron. Then they made balance patch which make them much weaker. Now they are more underpowered as overpowered.

They were overpowered because they got so much from ruins, which gave them very fast start.

And the extra scout with map revealed can be huge.

Spanish can still get a really quick start with those ruins :)