"Isn't that difficult?"
It shouldn't be, but I typed 難しくないですか and it was marked wrong. I've reported it (11/28).
難しいですか？would be closer to 'Isn't it difficult'.
When this course uses specific pronouns (He, she, I, this, that) it expects direct translations. Otherwise, 'that' becomes 'the' and 'we' becomes an unknown individual, etc.
In spoken Japanese it might be excluded based on context, but with the assumption that the listener understands to what the speaker is referring.
So, is "難しいですね?" not okay here because it would be more like "[that or it] is difficult, isn't it/right?"
Your sentence is perfect, but which one is good, better or worse really depends on the context, which is never given in Duo.
I would say that generally, your exampme is not correct because it implies that you have also tried "that", thought "that" was hard and want to gave the other persons confirmation that "that" is indeed hard.
The ね always imply that you have experience doing it. Think of it as a "right" in "Isn't that hard, right?"