The UI glitches I am experiencing:
- Light mode: system wide transparency issues.
- Dark mode: There is a slight unsmoothed black frame around the windows (most visible at the corners, especially in front of light background - I checked this on a supported machine, it should look a bit more refined). Some built in apps have visible artifacts (like the left upper and lower left corners of the Messages app). Also, some non-native apps which use transparency but does not support dark mode (1Password 6 for example) will exhibit the transparency issues.
So neither light mode, nor dark mode is perfect. I'd say dark mode is absolutely usable, but there are constant subtle reminders here and there that the system is in fact not supported.