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They where trained by Kami though, since Kami is a god you'd expect them to get a lot stronger.

Dude Goku used the kaioken, 3 super saiyan transformations AND the holy water.
Piccolo fused TWICE to catch up.
Gohan got his power unlocked twice.

Vegeta is the only one who never got a powerup that wasn't by his own merrits.
Well he did get the majin power up, but that was a midlife crisis. So make of that what you will.

I'm of the opinion Vegeta already had ssj2 before becoming majin. But you're still right in that it gave him a power boost. I just thought the power boost wasn't the primary reason why he did it, but his midlife crisis was.

But hey, you've got a point, my bias is showing.

I'm just pointing out that the humans have NOTHING going for them. Nameks can regenerate, Saiyans get zenkais and transformations, Freeza is just born with an absurdly high power level (and even he gets a transformation) Majin buu can come back unless you destroy him completely and Cell got the best of every race.

The one thing humans do have is their understanding of and manupulation of ki and their excellent techniques.

Besides mystic water is highly leathal, you need to be an expert martial artist to even consider drinking it, as Korin equated it to poison in the manga. So it's not like it's a gift with no strings attached.