There have been several problems with macOS Catalina so far that have caused issues for Mac users. One of the first issues that surfaced had to do with popular DJ software that runs on Macs no longer working with Catalina. Other software and apps have been unable to operate due to Catalina being the first version of macOS that has completely dropped 32-bit support.
Mac users running old plug-ins and other software that is infrequently updated may find critical software no longer operates. The Catalina update ditching support for 32-bit apps was no surprise; Apple announced that it would happen about a year and a half ago. One big issue is for people on macOS who are using older versions of Adobe products like Photoshop. The older versions are 32-bit and don't function with Catalina, and since the uninstaller is also 32-bit, the software can't be uninstalled.
All pre-Creative Cloud versions of the software are 32-bit; Adobe recommends that users of that software who rely on it not upgrade to Catalina. Newer versions of Photoshop also have issues with incompatibility with file naming issues, plug-in verification issues, and video rendering trouble. Adobe has warned that droplets, ExtendScript Toolkit, and Lens Profile Creator will fail to run in Catalina.
A potentially more significant issue than software that fails to run has to do with potential data loss in Mail when upgrading to Catalina. Michael Tsai, the developer behind Eaglefiler and SpamSieve for Apple mail on Mac, has written a warning for users upgrading to Catalina. Tsai says that he has heard from several users that updating Mail's data store from Mojave to Catalina sometimes says that it has succeeded, but when the users look closer large numbers of messages are incomplete or outright missing.
Users are also missing message content when moving email between mailboxes. Tsai says that there are issues for some users via both the drag-and-drop and AppleScript message moving methods that can result in a blank message only containing headers. He warns that if the message was moved to a server mailbox, other devices see the message as deleted. Since that eventually syncs to the Mac, the message ends up deleted there too.
It remains unclear if the email issues are Mail bugs, are due to problems on the Mac, or are due to issues with the mail server. Those thinking about upgrading to Catalina may want to wait. Users wanting to see all software on their Mac that won't function with Catalina can use Spotlight search on the Mac to open System Information Tool, scroll to Software, and click on Legacy Software.
All incompatible software will be listed at the top of the window. Users who decide to hold off on the Catalina update will want to go to Settings, Software Update, and unclick "Automatically keep my Mac up to date." That will prevent the machine from installing Catalina automatically. Not all is bad with the update, Catalina adds Touch ID and Face ID iCloud logins.