I agree. I'd add that a lot of people overlook the complexity of English idioms and vocabulary. It is easy to pick up the basics of English, it is much more difficult to get to proficient level for writing a book for example.
Imho as a native German I have the impression that with German it's the other way around. In the beginning it's hard. But once people attain a basic level there is not much to learn anymore.
Absolutely. When I was at school the only grammar I learnt was in Latin, French or German - wasn't taught in English. But as native speaker you read or listen and absorb the "rules".
Had a French girl friend for a time who was studying English and linguistics at Agrégation level (masters) - she would sometimes ask me was it better to say X or Y in English. I would usually prefer one because "it sounded right" but often couldn't come up with a reason as to why. Then she might tell me the rule according to her professor - and I almost always found them very convincing!
Compare that to English, where the mastery of the tenses (i.e. when to use which tense) is essential. Distinguishing the use of the simple forms from the progressive forms can be tough for non-native speakers. And then there are all the different past tenses: simple past, present perfect, past progressive... in many cases it sounds really strange when these get mixed up. Think of something like: I have gone to the store last night. Or: He never was in New York. (When the speaker means that he has never been.)