Oh there’s no question that Classic Mac OS crashed a lot. The underlying technology left a lot to be desired. That’s really beside the point though. None of crashes were caused by the UI design. They were due to a lack of protected memory and corruption of preference files, along with the ad hoc extensions system you mentioned. These problems could all have been solved without abandoning the truly easy-to-use UI.

Modern macOS continues to have a lot of very difficult-to-troubleshoot problems. They don’t usually cause the system to crash, but they’re very annoying to deal with nonetheless. A common one I keep having is random processes using 100% CPU for no readily apparent reason. Take one look at Activity Monitor (or Task Manager on Windows) and you’ll see dozens or even hundreds of processes running in the background. How the heck is my dad supposed to troubleshoot that? He won’t! He’ll notice his computer running hot and battery draining rapidly and ask me to figure it out for him.