> It is not that different from German in this matter.

I've met several Germans who spoke Russian fluently, none of them has really mastered the instrumental case, not even a friend of mine who worked at the German embassy in Moscow. Although you might say it's a minor grammar difference, this particular grammar case seems hard to grasp for people who are not accustomed to it through their native language.

Also, from my personal experience, quite a few Germans who learnt Russian had a real struggle understanding the concept of perfective/imperfective aspect.