The issue with low fps is not just lag, stability of the inputs also matter. (Average lags can be fixed with offsets, but not instabilities).
(Hypothetical) Example: A player plays a map, every hit and release he does is within 10ms early and 10ms late. The previous values with input device delays and instability already accounted for (but not osu! process input handing delays and instability).
If playing with very high fps: The player gets 100% Rainbows.
If playing with 60fps (and using -8ms UO to compensate for the average lag): Some presses that were 9ms late might be registered as 17ms late, or some presses that were 9ms early might be registered as 17ms early (if not using UO for compensating for the average lag, early hits would be always 10ms or less early, but the amount of late hits would increase in a way where things are likely to be even worse). As a consequence, the player doesn't get 100% Rainbows.