I wouldn't say "many". The only holdouts I know of are pro audio/video users with interoperability concerns; pretty sure they aren't the target market anyway.
But after doing a clean install of macOS 10.15.3, everything’s been fine.
I expect 10.15.4, currently in beta, to be as stable as any other version of macOS at this point in its lifecycle.
Turns out that Sidecar doesn't work if your mac is connected to a VPN, even if you are connected to your iPad via USB with its wireless turned off.