I don’t get why people keep saying Korea are dull. They have some really interesting adjacencies going on, do interesting stuff with Governors, and because they have such a narrow focus (in principle) you have to think creatively to use their bonuses to achieve something other than science.
The real problems are not really a problems with them at all. Instead, the core problems are (1) science is too powerful, and (2) science is too easy to get.
Campuses come early, there’s nothing to stop you spamming them, they give a metric tonne of science, you can multiply that science multiple times over with City States and Cards, and there’s no downside to having lots of science only upside (eg you only require power if you build powered buildings, your pop never need more amenities because of rising expectations) and the upsides are beyond awesome.
Korea are a bit like Hawkman in the DC universe. “Dude, what’s your power?” “I can fly.” “Dude, we can all fly”.
Sumeria, Korea and a few other Civs suffer from similar problems. They are the best at doing something that is trivially easy to do and just flat out wins you the game. The Civs are totally fine - the game just needs to be harder.