It's not just HDR content. I was watching a 2008 BBC drama called Little Dorrit on a Roku in 1080p streaming, no HDR because that didn't exist when it was filmed. It was super smeary, had terrible blacks, tons of obvious banding artifacts, it was really bad.

I bought the 1080p Blu-Ray version from the UK, and even my parents standing 10 feet away from our 48" 1080p TV (without me explaining the problems with the Prime version) could tell the difference after about 10 seconds.