As someone who has been learning Japanese for a very long time, I can tell confirm that Japanese grammar is quite complicated, as others have said. One example could be that adjective clauses go before the noun, instead of after the noun, which could be hard to wrap one's head around it.
The other issue is that the sentence structure is different from other languages. Japanese language uses Subject-Object-Verb structure instead of Subject-Verb-Object - to put it simply, sentences end with a verb. It could be a problem for a beginner, or even when you get to the intermediate level, where you could start hearing 3 complete sentences before getting to the first verb.
And another potential issue is that there are multiple levels of politeness. Each level changes how you say verbs and some other words. The highest levels even change some verbs to different verbs with the same meaning. But this is mainly issue only when you are planning to live there or work in Japanese company.
I don't know much about Mandarin Chinese other than that it uses different tones to pronounce words. And if you are like me, who doesn't hear those tones, it could be impossible to learn.
I believe learning Japanese pronounciation is going to be easier for some people, but it really depends where you come from. I never had to learn Japanese pronounciation because it is almost identical to my own (slavic) language.