[wrxtasy]

Thx for this explanation, wrxtasy. So Android is really a dead stick if one wants a good, not only Kodi but just generally speaking all round solid experience across apps, with auto switching for refresh rates, hdr-sdr, Color space, Et al, Unless you use players with custom roms such as zidoo, himedia, etc. but my understanding is those use android OS not the atv os, so you need an air mouse or the likes, is that correct?

You will only get HD Audio, 4K HDR & 1080p 3D video playback support on the above players when using their custom Media playback Apps. Kodi is used as a front end only for those external Apps. They are strictly speaking not even Kodi - HD Audio, 4K HDR & 1080p 3D players, but a dual hybrid setup.
There is also no DRM and HDCP for Netflix and Amazon Video Apps usage with Zidoo X9S and Himedia AND they need an Air Mouse for those Apps.

How about amazon firetv 4k? Since it is based on Android, is it plagued with similar issues for non Kodi apps or is it better than the shield ?

It's likely to have better colorspace management vs the Shield for Netflix and Amazon Video streaming Apps (not Kodi) but everything else like Kodi passthrough audio and refresh switching will be inferior. The HDR capable 2017 Fire TV Gen 3 is a AMLogic S905Z - it will also be slower than the Shield or Apple TV 4K.
It's probably fine for Netflix and Amazon Video streaming usage which is exactly its target market.

Please re-read the following post to see what the Apple TV 4K is capable of with ALL Apps:

Auto frame rate switching & dynamic range matching - 4K (HDR10) capable Hardware (click)

Amazon Video is also integrated properly into the Apple TV App (click), so you get Siri voice searching and more.

Also remember the ATV 4K actually has a Dolby Audio licence. If you have a DD only Soundbar / TV you will still get 5.1 DD Audio from the DD+ Netflix and Amazon Video Apps.
Not so on the Shield which can only passthrough DD+ from those Apps to a DD+ capable HDMi connected Audio receiver due to having no Dolby Audio licence. (yes I still shake my head in disbelief)