Waves, Toontrack, Arturia, Universal Audio, and Native Instruments plug-ins, virtual instruments, and hardware doesn't work, and that's only a small sampling of companies off the top of my head. There are dozens of others. Press releases have gone out saying their products are not compatible or only partially compatible with Catalina, and updates are forthcoming. While Logic offers quality, built-in plug-ins, no professional is going to limit themselves to the options Logic provides. Heck, I'm only a bedroom musician, but pretty much every plug-in I own doesn't work under Catalina, nor does my interface.
For those with questions on compatibility, Sweetwater has an excellent listing:
https://www.sweetwater.com/sweetcare/articles/macos-10-15-catalina-compatibility-list/#a