I have used Duet or Astropad for making iPad Pro as a second display. Then I tried to use Apple’s Sidecar which is a new feature of Mac OS Catalina. By the way it seems too unstable.
I frequently meet a freezing issue, a black screen and so on. Eventually I changed to Duet Display which provides much better reliability. Also, it provides more high-end resolution. But I don’t think whether or not this feature needs. Because the size of texts is too small to read.
I feel that Apple makes their product quality worse since Steve Jobs was gone. As a long Apple fan, I feel disappointed.