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It depends. On older devices there isn’t much testing of newer versions of the kernel so they can be more broken than older version.
Case in point, recently on an old laptop (~12 years) I noticed video performance was really bad which I later found out was due to modern distros defaulting to the iHD
intel graphics driver. But iHD
is only supported from 5th gen (Broadwell) onward. So, on older devices anything depending on the graphics driver for hardware acceleration (like video decode) fails and falls back to software rendering.
Distros ship with icons and themes system wide because apps running as root only load icons/themes that are installed system wide (you can check this by running an app as sudo).