[noggin]

Am I right in thinking that all Bluray's (including UHD HDR) only have 4.2.0 chroma subsampling ?

All TV Shows and Movies only have 4.2.0 chroma subsampling ?

So if you are only going to use the Odroid N2 to watch TV series and Movies, you will never ever use 4.2.2 or 4.4.4 ?

The Odroid N2 would never ever need to change to 4.2.2 or 4.4.4 ?

Thanks

You are correct that all consumer sources use 4:2:0 chroma subsampling within their compressed stream.  

However 4:2:0 video output was only added to the HDMI spec for 2160p59.94 and 2160p60 modes.  4:2:0 isn't valid or used for any other output modes - so 2160p30 and below outputs have to be in RGB, YCrCb 4:2:2 or YCrCb 4:4:4 mode, and can't be 4:2:0.  (4:2:0 has never been an option for 1080p and below output)  However RGB and YCrCb 4:4:4 are only supported at 8-bit (and thus SDR-only) at 2160p50 and above.

For HDR output at 2160p30 and below you can use RGB, YCrCb 4:2:2 or 4:4:4.  (NB - these are the most common frame rates for UHD Blu-ray movies, drama and docs)
For HDR output at 2160p50 and above you have to use YCrCb 4:2:0 or 4:2:2. 

4:2:0 at any bit depth is supported at 2160p50 and above.
RGB or 4:4:4 at any bit depth is supported at 2160p30 and below. 
RGB or 4:4:4 at 2160p50 and above are SDR-only.

4:2:2 12-bit is the only HDMI 2.0 chroma subsampling format that is supported at all 2160p frame rates.  However not all displays support 4:2:2 12-bit, some want 4:2:0 for HDR 2160p50 and above.

(4:2:2 is the standard used in almost all broadcast studios and editing facilities - hence it's widespread used in HDMI, even though consumer video 'on disc' or via OTT streaming services like Netflix, Amazon Prime doesn't use it. RGB and 4:4:4 offer equal bandwith for all video content - with no reduced chroma bandwidth - so are offered for when displays are used to display non-consumer video - such as PC use, where you don't want the smudged or smeared text that 4:2:2 or 4:2:0 would deliver.)

TL;DR - you can't run 4:2:0 HDMI output for all frame rates and some displays don't support all HDMI 2.0 options - so being able to force subsampling can be useful.  For UHD HDR Blu-ray movie replay - 4:2:0 HDMI output isn't an option for the vast majority of movies UNLESS you want to watch with 3:2 pulldown etc.