The UX is fine for people who are used to a culture where they see massively information-dense, brightly colored, and spacially organized things all the time. There's a strong cultural correlation in UX design, and what works in one country often doesn't work in another. I've worked on a few websites that are accessed globally and it's common that a 'one website for everywhere' strategy yields poor results. You have to consider much more than just language when you go international.
Accessibility is a problem but that's due to browser developers not really considering other cultures properly. My Japanese team never found a good solution to that.
No, it's really not fine at all. It's not just websites, it's electronic devices in general. The UIs are very complicated with every conceivable function right in your face. My Japanese girlfriend even complains about it. Things like washing machines and microwave ovens even are needlessly complicated to use. (Younger) Japanese people love western web sites/services such as Instagram, Twitter, Google Maps, etc., and don't insist on a Japanese UI-style version of any of these.
Don't even get me started on how well Japanese websites handle non-traditional-Japanese names, or how banking apps that supposedly support English actually display most text and dialog buttons in Japanese.
There's a lot of great things about Japan, but the UX/UI here is absolutely terrible.
When I used NMR scientific instruments that cost hundreds of thousands of dollars in the 1990s, the Japanese (Jeol) machines had a far more straightforward interface than the German (Bruker) machines.
Accessibility is a problem but that's due to browser developers not really considering other cultures properly. My Japanese team never found a good solution to that.