[Klaus]

The problem with the adapter is the "box" at the bottom (that probably contains the electronics or whatever). Sony had a similar design.
For long tele lenses that may not be relevant but for smaller setups (thus for the majority of use cases), this is downright disturbing to handle.
If it was just an "extension tube" (no, BC, not in terms of focus), it would have been Ok but this is a no-go in my book.
But ... we are all different ...