Custom UIs by themselves are not the culprit. Apple itself is "guilty" of that, using special controls and styles for their own apps (including even the new Reminders in Catalina.)
But if an app on macOS puts a close button in the upper-right corner of a window or panel (like Windows does) instead of the upper-left, for example, or when an iOS app uses the Android sharing icon, I feel annoyed.
A worse and more practical example is when an app is missing some of the global system features I described in my previous comment, like common text field options (spelling, transformations, dictionary lookup etc.) missing from an Electron-based chat app and so on.
Custom UIs by themselves are not the culprit. Apple itself is "guilty" of that, using special controls and styles for their own apps (including even the new Reminders in Catalina.)
But if an app on macOS puts a close button in the upper-right corner of a window or panel (like Windows does) instead of the upper-left, for example, or when an iOS app uses the Android sharing icon, I feel annoyed.
A worse and more practical example is when an app is missing some of the global system features I described in my previous comment, like common text field options (spelling, transformations, dictionary lookup etc.) missing from an Electron-based chat app and so on.