HDR support on Apple TV is fairly hit or miss depending on the player you are using.

HDR10+ is supported in some players (infuse) but not others (plex) and will fallback to HDR10. Profile 7 DV files do not play properly on Apple TV period even though it will display the DV logo (though frankly very few devices can do this). Profile 5 and 8 DV files will play properly depending on the player and the content mapping level in the content (2.9 is well supported but 4.0 causes HDR10 fallback or black screens in some cases). Apple TV also doesn't support HD audio passthrough great.

And now that the Nvidia Shield Pro is the same price, it makes the Apple TV an even tougher sell.