Too much assistance makes riding at slower speed uncomfortable.
For instance, if I select 15 km/h speed on a flat good road because of any reason (people walking there, nice views around, staying with the group that picked such a velocity, etc), full assist accelerates me too much. To slow down, I need to stop pedaling, then the velocity drops. Then I pedal again, the velocity increases and goes over the desired limit again.
These rush/slow iterations are annoying, it is much more comfortable to select the level of assist that matches the needed speed. Do not let the stupid engine with no eyes to decide which velocity is the best for particular road.
I think it is not a simply "bad controller" issue. There is certain amount of power the human can comfortably contribute without thinking about and much less may be less comfortable. Turning pedals with no resistance yet precisely at required speed needs more attention. In some degree this can also be fixed by shifting to the higher (not lower) gear - takes less power from the engine at the same speed. But gear alone seems not good enough for all cases.