Netflix also always works in HDR (or Dolby Vision) mode. So SDR content is send as an HDR signal to your TV and does not use your SDR preferences.You are correct about the Bluray part. No Dolby Vision for Bluray discs (yet). Besides that it works as you would expect.
Netflix does support HDR, Dolby Vision and Dolby Atmos, but has some disadvantages compared to Apple TV:
- Always 60 Hz. All 24 Hz content will be converted to 60 Hz. Some older TV's (like 2016 LG TV's) do not support Dolby Vision in 60 Hz mode.
- Always Dolby Atmos. 5.1 content will only use 5.1 speakers while others stay silent. No upmixing possible unless you switch to 5.1 uncompressed in Xbox settings manually.
I have no idea about Amazon Prime but I assume they have the same limitations as Netflix because these are true for all Xbox apps.