After Apple released the official version of macOS Catalina and I tried to use Sidecar with my iPad Pro. It seems okay. But it doesn’t work sometimes. For example, sometimes iPad Pro screen became just a black screen and I have to change the display option between Mirroring and Separated mode.
Today I updated the latest Mac OS Catalina and iPadOS 13.1.3 and now it feels more stable than before. If you have any issue like me please try updating your Mac OS and iPadOS.