Both my Japanese and Mandarin are bad, here are my thoughts:

1. When you’re learning, it’s sometimes hard to tell where one word in Chinese ends and another begins. Particles and okurigana in Japanese help a lot.
2. lol my ability to hear tones in Mandarin is still, somehow, below random chance
3. I don’t understand how people say Chinese grammar is easy. It isn’t.
4. Kanji/Hanzi used in Japanese and Chinese to express one thing may be completely different, ex 日 vs 天. also some characters like in 図書館 and 图书馆 have been modernized differently.
5. learning simplified characters was confusing to me. many “new” radicals.
6. kokuji exist, idk. don’t think 辻 or 働く exists in chinese. iirc some japanese words were loaned back into chinese, but I can’t list any rn.

P.S. This video shows that Japanese just barely understand Chinese writing. It’s worth watching IMO. https://www.youtube.com/watch?v=rzJqXd-1dEU

P.P.S. I know enough Japanese to read easy manga and I guess I could pass JPLT N3 if I studied for it. Reading comprehension tests in N1 are a bit too much.

P.P.P.S. I sorta screwed up, 2 mistakes. The online N3 test sample seemed easy. Got text about allicin in onions in one exercise, which was…informative. One listening exercise described a conversation between friends, where a person comes to apologize for wrecking a page in a book they borrowed…and doesn’t return it…Who tf does this? I don’t think you can pass these by just knowing Hanzi, sorry.