In his own universe he isn't overpowered, but compare him to others.
Regardless, I don't like the comics or animated movies from Marvel or DC because of how overpowered the characters are.(Before someone jumps on my dick, I'm sure they may not be OP in their own universe) That's why I liked the live action movies. They made the characters seem much more believable and realistic.
Same with DBZ. While I still see them as OP, they aren't too over the top and have many limits, like Goku being able to lift only 40 tons in his base form and having a thrown rock hurt him while a Full Power Super Saiyan.
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Bearing in mind that the rock was thrown by the most powerful human being in existence, who is capable of busting planets.
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Compared to a Super Saiyan, he is pretty weak which is why I mentioned it. Hell, he is weak compared to Goku's base form. It is honestly impressive that he could even do damage to him. I'm just guessing that the fact that Goku didn't expect a rock to come flying at his face is the reason it hurt him. No time to prepare.
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True. Every superpower displayed by Goku was because of ki. To get super strength, he charges ki into his arm and punches. Super speed, he charges ki through his whole body and run. Flight, same deal. If he doesn't focus his ki he's basically just human. A really strong and well-trained human, but human nonetheless.