Slightly different perspective, but while I lived in Japan I did the JLPT as a native English speaker.

Among my friends: Koreans have no problem with Japanese grammar, struggled with writing/reading. Chinese no problem with reading/writing, struggled with grammar. English speakers.. struggled with everything but pronunciation hah.

As per Chinese characters. There are a lot of subtle differences: 'Traditional Chinese' is the original. 'Simplified Chinese' was done by the Communist party, is the standard in Mainland China. With notable exceptions in Hong Kong and Taiwan

For Japanese, there was simplifications in the 50's:

https://en.wikipedia.org/wiki/Shinjitai

But for the most part they're pretty similar to the Traditional (closer to traditional than simplified)

I'm not super familiar with Hanja(korean), but I assume they are the equivalent of 'Traditional' Chinese.

Then you also get into subtle drifts in meaning. My favourite was:

手紙 The characters mean 'hand' and 'paper' respectively.

In Japanese, it means 'Letter'

In Chinese it means 'Toilet paper'