HDR games is fucking baller on the steam deck. I'm legitimately thinking of switching to kde from sway so I can take advantage of it on my new OLED monitor.
My TV and PS4 Pro have HDR. I'm sure it helps make brightness better, but it just makes everything look yellow.
Also, I don't even think my TV's HDR works with its apps. I distinctly remember House of the Dragon and trying to see something. I accidentally closed the app and reopened and suddenly it was super clear. It's like it turned the HDR on (or off) and suddenly everything was visible in an otherwise dark scene.
Quality of HDR is very much dependent on the TV you have I think.
I'm still rocking a 2017 LG OLED which are considered pretty good, but as you go down into LCDs and the cheaper brands, you'll probably take a hit on image quality. Some TVs used to have a yellow pixel as well as red blue and green, so could even be that.
HDR is less about the brightness (although they are brighter than older TVs) and more about colour and brightness accuracy.