In my load work with the .264 Win Mag, I have found Quickload data to track closely with actual data, using RL-33. That has not been the case with Retumbo.
When I did my initial load work to confirm Quickload predictions in my rifle, I found Quickload predicted powder charges approximately 3.7 grains higher than actual for a given muzzle velocity. For example, QL predicted 73.67 grains of Retumbo would produce 3153 fps. During initial testing, 70.0 grains of Retumbo produced 3156 fps.
In the short time I have used Quickload, I have found it to be a valuable tool. It can also lead one dangerously astray, if it is not used with caution.