The difficulty depends on personal understanding, just like going to school. Some people get good grades, while others get poor grades. Generally speaking, if compared with other musical instruments, it is easy to learn.

As for what you said, it is natural that not many people learn Qin Qin, Qin Liu, dulcimer and three-string clarinet. Compared with piano and violin, these instruments are originally unpopular instruments, but nowadays mainstream orchestras, especially symphony orchestras, are usually more difficult to see such instruments except for some special tracks, so they are not used much, so people who learn them will.

I hope I can help you and adopt it.