In my experience migrating to another provider from iCloud, this hasn’t been a significant issue. Live Photos in particular are not really proprietary in the sense that they’re implemented in an extremely simple way that basically every photo tool understands. ~~Slow motion videos are also not proprietary, they’re just a plain video file.~~ <<< edit: I think I’m wrong about slow motion

> Slow motion videos are also not proprietary, they’re just a plain video file.

I haven't looked into the implementation details, but Photos lets you adjust the section of the video that is played back in slow motion. I thought if you share a slow-mo video, it gets re-encoded to bake this in (i.e., one second at 240fps gets exported as four seconds at 60fps).