How does ki work?

Dragonballz is very VERY inconsistent in the way ki energy works. Vegeta's Gallick gun can destroy a planet when the user's power level is only 18,000 but characters with 1,000,000+ power levels have their energy blasts just pockmarket the ground. Additionally dueing the Cell Saga, Goku was afraid to put that much power into the Kamehameha Wave, yet in Resurrection of F, he fires one from the air at Frieza no problem. Does ki follow what the user wants to do, instead of physics?

If a character invests too much power into a ki-blast then he'll lose control over the collateral damage it will cause. That's why Vegeta's Final Flash and Goku's Instant Transmission-Kamehameha would've blown up the Earth if not redirected.

But neither of those blasts were as powerful as Gohan's Father-Son Kamehameha. So yes, there is a degree of control involved.

You cannot judge the potency of energy attacks in DBZ by the amount of collateral damage they cause. If that were the case, then Roshi would be one of the most powerful characters in the mythos.

I think Ki works like Mana. Means, the Abilities of the Chars have a base potency that can be further amped by their Ki. Those Abilities have limits for example, the destruction they can cause. If you use an Earthbuster against another char you have to spend the necessary Ki, if your opponent has more Ki, he can block it with his Aura or with an attack. Once you spend it you will need time to recover it. That means the higher your PL the more Ki you can spend to amp your strength, shields, blasts etc.