The problem with Denon and the other mentioned AVCs is that, by using the only HDMI 2.1 available chip at the time means they cannot use 4K/120Hz or 8K/60Hz if the data is uncompressed. It‘s not the Xbox at fault either, as they chose to pass uncompressed data streams out.
It is actually the other way around. The 4K/120Hz output from the XBox Series X is compressed. It is why you don't have this issue with the PS5 due to the fact that it outputs 4K/120Hz as an uncompressed data stream.
HDMI don't force manufacturers to include everything included within the new version specifications. It is why HDMI org dislike manufacturers actually stating HDMI versions and prefer they state capabilities or associated features that the device is able to provide or attain. Most of the new spec is actually asperational and some of it may bever come into use becfore HDMI is once again revised. They have gone further than they usually do though this time around in order tpo try make this version last longer.
Support for DSC 1.2 compression is optional and not mandatory.
Anyway, Sound United never specifically stated that they'd be providing support for both A and B signals so they do have a get out of jail card. Yamaha on the other hand …
What is wrong and very worrying is the fact that these products are churned out year upon year and do not appear to be tested. THere's never been a year were new models haven't exhibited some kind of major issie that is very apparent when the hardware is in use. If the product wasn't ready and couldn't be tested then they simply should of ignored their announced schedule and not launched the products. No one really needed them. It isn't as though they were ventilators!