I installed macOS Catalina yesterday, despite years of warning others not to immediately download new OS versions. My sacrifice is for the greater good.
There are some big changes in Catalina that affect applications. Support for 32-bit apps is gone, so some of your favorites may no longer work. And security is amped up, which adds overhead as you're setting up the system.
If you've been using macOS for years some things haven't changed. For some reason, as it has for years, logging into iCloud - which you'll have to do to manage your Apple devices - is still a royal pain. macOS insisted on a new iCloud password, and then, when I went to use it, refused it about 20 times. Same password worked fine on iOS and iPadOS, but not macOS until I restarted the system a couple of times and let it sit overnight. Why Apple, why?
But that's not unique to macOS Catalina. It's been an iCloud problem for years.
No, the real surprise was how many of my 300+ apps weren't compatible with Catalina: more than 10 percent. Now many of these were free, but useful, utilities, such as the Blackmagic Disk Speed Test. I hadn't updated it for years, and when I went to the app store to do so, the app store insisted that since it was already installed, I didn't need to download it again. Oh, bother!
No disk speed tests in the offing, so not critical. But my entire install of Microsoft Office 2011 is now kaput, which, had I been in the midst of an Office heavy project, would have been most unwelcome. But I long ago switched to Apple's Pages, Numbers, and Keynote - which read and write their respective Office file formats - and much prefer them.
Other kinds of affected apps, besides utilities, are media players, file managers, writing apps, and some website building software. I haven't always been diligent about updates, so your mileage may vary.
There are other hassles, such as granting permission for apps to accept keystrokes, or access files, that are part of Apple's focus on security These are Good Things in the abstract, but they add time to entire upgrade process.
Inventory your apps
But how do you know which apps are 32 bit? Easy.
Click on About This Mac under the Apple menu. In the window, click on System Report. In that window, under Software, click on Applications. Now wait. On the far right column click on 64 bit, which will group all the apps by either 64 bit or 32 bit. The 32 bit ones are toast in Catalina.
The Storage Bits take
If you are in the middle of a time critical project do NOT upgrade to Catalina yet. You never know what might break, and how lengthy or costly the fix might be.
IBM mainframe managers used to wait six months after a new release to put it into production. macOS doesn't need that much time - you aren't running corporate payroll on it, are you? - but if you have no pressing need, wait for macOS 10.16.1, give yourself a solid four-hour block of time, and then go for it. It's a pretty smooth upgrade as these things go - Final Cut Pro works! - but there's a lot of niggling housekeeping to get through.
You'll enjoy the new features - more on them later - a lot more if they don't first delay a project. Relax, the update will be waiting for you whenever you're ready for it.