That is very situational, and who needs UU when you are 1-2 eras ahead of competition ? That is usually the case with Maya.
It's true in the long term Korea will get more GP but it will take quite some time to get there.
Rationalism is also accessible to Maya, and because of it's jungle boost actually Maya benefits from it even more than Korea. Golden Ages I agree but it will depend much on things like Chichen Itza which you can't secure reliably cause your tech lead is not high enough in Renaissance.
Regarding scaling, it's not true at all. Kuna gets so many buffs over time that in the endgame it's almost like a mini Academy. And you can go really wide with Maya - unlike scientist slots, your Kuna works well in every expansion. Just to give an example - my last Maya good game where I had 8 cities most in a jungle/forest mix with a mix of tradition/progress/fealty/rationalism/industry, around turn 200 my science per turn reached 500. My last Korea game where I expanded to 6 cities I reached about 300 science per turn (which is still good result in my book, just not really close to Mayan).