Even the "just works" experience isn't great because it's just not intuitive to setup. There's no confirmation that yes, your iPad and this other machine are connected. It either fails silently, or you realize you can move your cursor over. But where do you move your cursor? You also need to setup Sidecar to tell it where your screens are located. The only way I got it to work was to look through third-party articles. I'd love to see a dropdown list of what machines I'm connected with.

In addition, going from my Mac to an iPad is laggy and it had the caps lock flipped between each machine!

What is really fun though is Synergy, which a few years ago was running a Mac, a Linux box, and an O2 all from the same keyboard and mouse.