17's power is bullshit because Super doesn't even bother trying to rationalize anything anymore. Yeah Z had silly power scaling but they at least bothered to make up reasons for why they received a dramatic power boost, and why it was something that they couldn't have achieved previously - which yes can still be something stupid like Gohan sitting on his ass. At lease the Kai ritual was SOMETHING that couldn't do before, and that's basically how I accepted the SSG ritual.
As far as we know 17 just..... protects an island and fights military guys which is exactly on par with the warriors of the Dragon Ball era. "They trained" just isn't a satisfying answer, all warriors train but can never allowed to advance for it - even RoTaS training is portrayed to have limited gains.
Some people can just accept that it's all bullshit and sure you're right, but Super rationale really doesn't even try and it's not at all the same. And when Super has to juggle its frame of reference around Blue being a godly and terrifying power that literally frightens gods from other universes, to being some scrub trainin' form... it's a bit of a mess to watch at times. Really wish they didn't forget SSJ3 exists and that they'd quit trivializing Blue.
Everyone has different potential that can come from training so everyone will get different gains out of it
Android 17's training potential > 18's potential > Freeza's potential > Gohan's potential>Gotten and Trunks' potential> Goku and Vegeta's potential
Buu probably has even higher potential to Android 17 so if all these people are this strong with training then it makes sense that EoZ Goku would want to train Uub in 5 years because he realizes that even someone with Kid Buu's potential if trained(especially by him) would be a monster in power