Thanks for the advices. They are good orientation, even if we can find exceptions. But I have to break a lance in favour of parentheses.
I concede that most are unnecessary, but for, parentheses are a graphic representation of another plane of speech. An intervention of the author, accotation or just a subtle change form the flow of discourse. In your example, for instance, I don´t read both versions as equal. The second is pure enunciation, speaking about a hard course. The first is about passing the corse and, from a different place or point of view, someone whispers that the course is hard.