It does HDR10 & Dolby Vision, but not HDR10+ or HLG. Iirc there's some strange app-dependent (non-)support too.

I have the older (not 4k) Shield, I'm just using TV apps since upgrading the TV, since I couldn't settle on anything else. Tried and didn't like the 4k Fire Stick.

It doesn't do AV1 in hardware. Not yet necessary today, but nice to have for future proofing.

Who really needs ATMOS, wasn't that only marketing fluff?

I use it with Kodi and Jellyfin as well use streaming apps such as Netflix as well as geforce now. But its processor is old and it shows its age when going through movie collections. It does not support VP9 and now the new coming AV1 codec. So for someone getting a new device today I would not recommend it as the price premium they are asking for a 7-8 year old processor based device I feel is not worth it. With a Jellyfin or plex server you can play most of your media on your smart tv directly or you can get a $50 google tv or amazon fire stick

Shield does support VP9 as such; what it doesn't support is VP9 Profile 2 (i.e. 10 and 12 bit decoding).

Jellyfin user here. Started on Roku, moved to a Shield because I needed more codecs (my server's too weak to live-transcode anything above ~720p) and wanted to get there cheap.

Should have just gone straight to the AppleTV, which is where I ended up. Better UI, not shitted up with ads, less jank and crashing (friggin' Android), and none of the problems I'd see with playback of certain combos of audio and video codecs on the Shield. If not for the weak codec line-up, even sticking with Roku would have been better than the shield (yeah, it's shitted up with ads, too, but not as bad—hell, the Apple TV kinda has some, but again, less bad).