Speed. The processor is far beyond anything else on the market. It makes everything feel so responsive that it’s hard to go back to anything else after using it.
No ads or bullshit in the operating system. You can opt out if data tracking. The Home Screen is just a grid of apps. Individual apps may have ads in them but the OS itself feels like it respects you.
Reliable frame rate matching. 24fps video content outputs as 24hz signal to remove stutter and frame skips etc. Other devices offer this but are less reliable.
Infuse. Great app for playing back your own files. Connects to Plex, Jellyfin etc and works extremely well. Makes Kodi feel ugly and clunky in comparison.
Good integration with other Apple stuff. Like using your phone as a remote. Turning it on and off using Siri. Etc
The screensavers. They’re seriously great.
Disadvantages:
Price.
Lack of customisation. There’s not much to tinker with under the hood.
Not easy to side load apps
No audio passthrough for playing Bluray atmos rips. Best you’ll get is lossless 7.1 audio.