A quick real world review of these two platforms:
CoreELEC: Everything just works. It's hard to actually appreciate how good CoreELEC is without first hand experience of another platform where things don't work. Really the only downside is the old kernel.
Raspberry Pi: (using a nightly build of LibreELEC as of Jan 2022, as the stable build is even worse)
- 1080i deinterlacing is a poor quality bob algorithm, causing static picture elements such as sports scores and news tickers to flicker up and down, and the perceived resolution looks like point-sampled 720p at best. In the stable build there is no deinterlacing support at all. I'm spoiled by the quality of the Amlogic deinterlacer, it looks indistinguishable to the native deinterlacing in my TV.
- Random frame skipping during playback, followed by changes in video speed to compensate. Timing is just messed up overall.
- Clips SDR white at 235, i.e. does not pass through whiter-than-white. (Black level was verified to be correct.)
- In SDR audio is not synced to video, and in HDR it is chipmunk-grade unlistenable with continuous buffer underflows. These problems are resolved by enabling the sync audio to display option, however this replaces them with a new problem of random changes in audio pitch.
- UI elements in HDR have saturation turned to 9000% - e.g. mild yellow shades become retina-searing red. Images are reduced to single shades of max saturation primary colours. The UI is not needed much in HDR to be fair and is already blinding, but this makes it even more blinding.
- HDR gradient banding in test patterns noticeably worse than the Amlogic or TV's built in player. This is with the 10/12bit fixes in place on the Pi.
- Could not get CEC remote input working on the Pi, even though CEC itself was communicating to some extent (verified with cec-client)
- Hardware-wise, the Pi is lacking the out-of-the-box features of Android TV boxes. For the same price they come with: power adapter, eMMC, case, heatsink, LCD display, IR receiver, working USB implementations, etc. You could nearly double the price of the Pi to add these things at retail prices.
- On a positive note, the Pi displays heavy Kodi skins more smoothly