I love reading discussion about overrated/underrated artists because it´s almost all the time more about beeing overappreciated/underappreciated artists by the general public.
I agree with some of the mentioned artists regarding the amount of main roles they get in comparision to some of their more seasoned and gifted colleagues. Many times I thought the supporting cast outshone the main leads - not only in korean shows.
In South Korea looks are important to get a main role. I live in germany. Here the looks are less important but they help too.
Regarding the sometimes expressionless acting / stiffness - I think that´s a cultural thing and just a different way to act. I have seen many spanish shows too and there it´s the polar opposite. They seem to overemote but I think it´s just how their culture is compared to mine.
Nevertheless, despite all these thoughts of mine there are still some actors who should improve. IMO these would be: Cha Eun Woo (I like him but he can´t emote at all), Im Soo Hyang , Song Hye Kyo (I know, she is a legend somehow but I just find her boring), Park Shin Hye (feels always the same)