Symbian Belle FP1/2 - Best Compatibility
Symbian Belle/Refresh - Untested. Presumably the same as Anna.
Symbian Anna - Works fine with some errors on startup, close the errors, restart UCSOS and it will work fine
Symbian^3 - Bad compatibility, on top of the same errors from Symbian Anna, buttons are unresponsive and hard to press and sometimes disappear. Despite this, UCSOS still has complete functionality on Symbian^3 despite this difficulty.
The difference between Desktop UCSOS (Win, Mac, Linux) and Mobile UCSOS (Symbian^3): The mobile version misses out on the Image Viewer, 4 songs in the Music Player, a swap switching out the Nokia 808 PureView ad for a clip from Top Gear. This is due to there being no point in having a HD video to stream on these old phones. It lacks the Grass and Sky background present on the Main Menu, UCSOS runs at a lower resolution on Symbian^3 (640x360) and has far worse audio quality. Other differences which are not compromises include a light background (some S60v5 phones style) and a song swap switching Meat and Potatoes for I'm Waking Up To Us.
This paragraph correlates with the Further Notes one on the topic of RAM.. Most of these changes are due to Symbian Flash Lite 4 limitations being UCSOS will crash if it is too big to fit in the phone's RAM, on higher RAM phones such as the 808, it seems to be 25MB with no other apps open, lower RAM Symbian^3 phones such as the N8, it's somewhere around the high teens to 20MB. So in order for UCSOS to run on all Symbian^3 phones changes had to be made.
Further notes: Ideally, UCSOS should still run the best providing your phone has a high amount of RAM (Nokia 603, 700, 701 & 808) even if it is not running the latest Belle FP2 firmware, such as an unupgraded Nokia 603, this theory however, has not been tested. As for older Symbian phones, for example, S60v5 phones which support Flash Lite 4, UCSOS is unlikely to run on those phones however since they have far less RAM.