The most painful one for me right now is that Paramount+ has all their videos rendered with black bars top and bottom. So when I watch the latest Star Trek New Worlds and Star Trek Discovery on my 34" 1440p ultrawide, it literally has a black border all the way around the edge. Watching the same episode on Prime Video, it entirely fills my screen.
I've reported this to them a couple times over a year and they haven't done anything about it. I imagine, most likely, this is possibly some kind of combination of compatability with some devices that need videos rendered at a certain resolution and not wanting to have too many different encodes of videos. But it is quite annoying not to be able to have the video fill my screen.