The more G the less accurate the indication.
Repeating it doesn't make it true. Do you have any evidence?
The reason why we use the simple calculation of (airspeed (in knots)/10 ) +7 to obtain an angle of bank for rate 1 rather than simply select rate 1 on the turn coordinator (when using full pannel) is becuase of the inaccuracies of the turn coordinator.
The reason we use this approximation is to get an idea of what bank angle will produce a standard rate turn. When you use it, the turn coordinator will indicate a standard rate turn.