I wish that people referring to Japanese as "the most difficult language to learn..." could add the important caveat "...among major global languages". There are thousands of languages spoken on Earth, and some of them involve complexities that would daunt speakers of English or other Standard Average European languages more than Japanese. As just two examples, I remain daunted by the morphology of Skolt Saami and Ng…
The consistency in phonems in mandarin probably makes it a bit less demanding than Japanese, but I find that difference marginal on the whole.
(Studied Japanese 1y+, Mandarin 6 months)