I flew Korean Air once, from Hong Kong to Seoul, because Cathay Pacific was filled. It is pretty sad airline by Asian standard. The service is terrible and the seats were not comfortable. I would rate it somewhere around Shenzhen airline level, below China Southern. The plane is also very old and somewhat dirty. Personally I would not fly it again unless I have to. The CRH trains are far more comfortable of course than any airline so you can't really compare.
I have flown Korean air couple of times from Sydney to Seoul. I was more than happy, just as good as Singapore if not better.
Maybe Korean air puts their shitty service and older planes on the Chinese route? Gulf Airways and Emirates do the same with their sub continent services.