I've mostly consulted and worked with companies based outside of Korea, so I can't speak to onsite experience personally.

If you're wondering about work-life balance, in which Korea has a poor track record in general, it's going to vary wildly based on the team. The best is to try to get recruited from outside of Korea if you're looking to join a larger company. The startup scene is vibrant, too.

It's hard to give general advice without knowing your motivations, but feel free to drop me an email and I'll be happy to offer any advice :)