As far as I know, the main thing hamstringing native Debian support is that the binary blob required to boot any of the Pi's is not DFSG-compliant so Debian won't ship images for the Pi. The second biggest thing is that you still need a custom non-mainline kernel.
Kernel support improved a lot and we now have https://pignus.computer/ running mainline kernel. Graphics and camera support are still missing but we'll eventually get there with 4.5 kernel.
