They do have an adequate research and testing period. Bugs can pop up occasionally regardless, it’s just the way software works, especially when so many variables are involved. Code is incredibly complex, especially at that scale. Perhaps you should try maintaining an array of OS platforms with millions or billions of users, and see if you can make a perfectly bug free system…
No system, no matter how well funded or designed, can be perfect.
I honestly have to agree with
@fisherking, the idea that the older versions of macOS were somehow perfect and everything that’s come since is supposedly worse I find to be completely absurd… Modern OSes are far more capable… And look far less hideous…
I for one am glad that Apple has pushed their software forward and improved it.