It had some underlining points, that most people argeed with. That if you study a week with Mandarin vs Japanese you will beable to use your Mandarin less in the real world than if you studied Japanese for a week.
This is because the tones of mandarin combined with the bad pronounciation makes it really hard for locals to understand you.

However learning Mandarin at the start is easier because of the grammar where japanese grammar hits you like a huge right hook at the very start.However after that first bump of grammar(will last for a few months) it actually gets easier where the difficult of the tones will last for years.