Not entirely true. Didn't a half black woman just win Miss Japan last year or something?
Of course there were shitbags online talking trash, but that is to be expected nowadays, unfortunately. Seems like she was very popular among the general populace.
Pretty much any "half" is going to face stereotypes and diet racism, though a half-black half-Japanese person is obviously going to stand out more.
Most of my friends who have lived in both countries did say that they ran into more inconveniences in SK, despite the English level being higher and such. Especially restaurants and bars straight up denying foreigners service.
I've still never been denied service in my 10 years living in Japan.
The problem with apartment hunting is real though. It's fucked up, but as someone who has seen at least a dozen foreigners dip out of leases for various reasons I can sort of understand why that shit happens.