How good are they with respecting content protection flags?
I've thought about getting one for a weekend project, but very occasionally my AppleTV lies to me that it can't play my own home videos because my TV doesn't support DRM. So I'm uneasy about buying a piece of gear only to find out later that it's flaky in the same way.
I wanted to do some capture from a PS3, as another commenter mentions they set HDCP on all output (even games) and my Elgato capture card does respect this flag. A $15 HDMI "output splitter" will strip this however, and conveniently this also means that my game does not run through the capture card (with any latency that might entail - along with having to run the capture PC even when I do not want to capture) but rather goes console -> splitter -> tv which is about as low-latency as you can get.
Note that HDCP has multiple versions - your video source might support HDCP 2.3 while your TV might only support HDCP 2.2, and that's largely fine, most content will be allowed to fall back to the older standards (which can be broken). However 4K content/etc at the highest quality levels (eg from streaming/etc) often is not allowed to be delivered at the lower quality levels - Netflix will drop the resolution if it doesn't like the DRM chain it sees (eg no SGX enclave or insufficient HDCP level) and things like a BluRay player where there's no fallback might refuse to play at all (or might similarly fall back to a 1080p downscaled output, etc).
Home videos shouldn't have HDCP flags set at all, so it's hard to say what might be happening there, but it's possible there's just some broad-spectrum "if resolution == 4K THEN require_hdcp_2.3()" type thing. I would look at the particular types of content that it's barfing on and see if there's a correlation like resolution or source device and go from there. It's a bit of a pain for sure.
Also - while HDCP 2.2 and below are thoroughly broken even by garden-variety devices at this point - HDCP 2.3 is still pretty tight. There's a few (expensive) devices that claim to do it, but it's not something you'll find in the $15 tier of "splitters". If that's the requirement, figure on paying several hundred dollars. It's gonna be easier to figure out why the Apple TV is turning on HDCP and get it to stop than to break the HDCP.