> Also I'm surprised when people assume the grammar is difficult - to me, the "flexible grammatical structures" as mentioned in the article are a good thing because it gives you more leeway to make "mistakes."

That makes speaking the language easier, but it makes understanding sentences harder in the beginning, because you get a bucket of word soup dumped on your head and have to figure out what is supposed to go where.

Of course, articulating yourself correctly is the bigger (or more long-term) challenge when learning a second language, but in the case of Japanese you have a higher up-front learning curve than with most other languages.