I don't have problems with flat. I do have problems with unnecessary borders around buttons. In my opinion, space separates design elements enough and borders, beautiful as they are, with glass transparency effects and other bells and whistles, is just a visual clutter. And Liquid Glass design introduces quite a lot of new borders upon borders upon borders, in a style quite resembling an unholy love child of Windows Vista and flash websites of the 00s.
The could definitely do with cutting down some of the empty space and the “border stacking” problem that feels like old fashioned “div padding” stacking when doing html, though I feel like that trend started a few versions back in the flat ui design. Interestingly the iPhone version of Liquid Glass seems to have much less “extra” white space than the Mac version of it, which seems backwards because the iPhone should need/want more for larger touch targets. Or maybe it’s just less noticeable because it needs those large touch targets but also lives on a smaller screen.
I guess it's part of the trend now. Some time ago both Google and Apple rolled out very flat UIs -- Apple in iOS 7 and Google with Material Design in Android 5, and both where mostly flat. I don't know, maybe they were influenced by Microsoft's Flat UI that ultimately went nowhere. Yet, over time, Google began introducing more and more meaty designs with more aggressive rounded corners, eventually ending up doing everything very curvy. Maybe folks at Apple looked at that and wanted some of those curves, too. So previously the trend was to flatten everything, now the trend is the reverse of the previous.
