You can move windows without any decorations (press the alt key and drag the window); resizing doesn't really work, as there is nothing to handle the repaint.
Consistency is a matter of POV - it is not really consistent, if your decorations have different widgets, or even font rendering (Qt is quite specific in this) than the rest of the app. The apps themselves are not consistent anyway, so having consistent decorations, for the price of dragging entire widget library and font shaping & rendering into the compositor process is not worth it. That's on top of the synchronization.
Consistency is a matter of POV - it is not really consistent, if your decorations have different widgets, or even font rendering (Qt is quite specific in this) than the rest of the app. The apps themselves are not consistent anyway, so having consistent decorations, for the price of dragging entire widget library and font shaping & rendering into the compositor process is not worth it. That's on top of the synchronization.