During the circle to land part on an instrument approach pilot must :
1) see the landing runway at at all times during the circle and
2) stay within the prescribed distance radius as per aircraft category.
3) maintain prescribed MDA
4) keep speed bellow max for aircraft category
The combination of MDA, distances and speed during level part of a circling app protects you from all obstacles and terrain.
You are not obliged to see them - you just look for the landing runway - just as you do in a straight in approach.
It is an instrument approach as any other.
Visual approach s quite different - you must see the surrounding terrain and obstacles all the time and be able to safely avoid or overfly them. Speeds and altitudes are up to you.
So the determining factor is the ability to see enough of terrain and obstacles.
Now the question is: can you follow a straight-in segment of a published instrument approach until breaking out of clouds and then require visual approach instead of continuing with circle to land as published?
It is possible providing the pilot can see the terrain and obstacles.
ATC person is not aware how familiar ( or non-familiar ) is the pilot with the surrounding terrain nor what pilot actually sees from the aircraft.
By night with some rain showers acting as curtains terrain and obstacles are simply not visible.
Also Calgari accident shows that the ATC can be found to be partly responsible in case of an accident - depending on what the particular judge thinks about it.
I am not expert for procedure design but I suppose that something in the particular RNAV approach makes circle to land impossible.
So in this situation where there is no published circle to land, wx not ideal, night, foreign crew etc... ATC should be very reluctant to issue a visual approach clearance.
And pilots should not press them for one.
Day time may be different.