Sense, Nonsense, and Not-Sense

Yes, harmony and moving line parts where nobody else is following or playing / singing with you are the hardest. It requires Supreme Confidence from the player / singer that they are in fact doing the correct thing. And then, finding where the harmony fits into the piece overall is an additional challenge on top of playing something that nobody else is playing or singing.

When it all comes together, it sounds beautiful. Until then, though, it often sounds awful.