So, this is what I am seeing in terms of both Roku and Fire TV sticks:I just tried it again on another Roku unit and it worked okay. I still prefer using it on ATV4K units.
Roku Premiere:
4K UHD and HDR10 (reverts to HD when connected to an non-HDR capable UHD display)
Dolby Atmos (even on my non-HDR display)
Roku Streaming Stick+:
4K UHD and HDR10 (did not attempt to connect to my non-HDR capable UHD display)
Dolby Atmos
Amazon Fire TV Stick 4K:
4K UHD and HDR10 (I do not have a DV-capable display to test this on, but 4K UHD did show on my non-HDR UHD display)
No Dolby Atmos (only DD+ 5.1)