AMD graphics require a firmware blob for all modern cards [0]. It used to be that the firmware was only needed for 3D acceleration and you could run X/text mode without the blob just fine, but that hasn't been true for years (I think since HD6000 series in 2010).
It is my understanding that a lot of modern GPUs that are cutting edge ship with non-oss binary blobs, which goes against Debian's core principals.
Unfortunately, it means that Debian has poor support for hardware vendors that mandate these binary blobs.