I personally think it’s due to Apple engineers trying to do so much; I remember being impressed by the number of new features and changes introduced this year while watching the WWDC, but it looks like Apple couldn’t keep up its own enthusiasm.
Project Catalyst was poorly executed; I was disappointed that Catalyst doesn’t default to ‘HIG-respecting’ AppKit controls that correspond well with the UIKit equivalents (so that modal form sheets in UIKit are translated into floating windows, e.g.), and the inability to mix in AppKit controls and UIKit ones into the same view hierarchy (NSColorWell or NSPopUpButton doesn’t have UIKit equivalents), e.g.
SwiftUI is great in theory, but the tooling is immature, it’s not available to write any decently complex apps with them.
The iCloud changes were reverted right before Catalina’s release; looks like the iCloud team didn’t have enough manpower to fix all the bugs until the release.
It’s concerning, as one of the premise of macOS was that it works as expected, though that now the newly introduced parts doesn’t. I’m expecting a stabilizing release for macOS 10.16 akin to the stone-stable 10.14 Mojave, but I don’t remember 10.13 High Sierra to be unstable like this one.
[0]: For Linux users who don’t agree with this: I’ve never set up a Linux desktop system with all the hardware working without doing any ‘geek’ stuff like running some shell commands. And no, the fact that Apple only allows to use macOS only on its own computers isn’t an excuse. And no, even on well supported laptops like the ThinkPad, there are a furious amount of bugs that need ‘configuring’