I haven't seen the same issues with my RF100-500.I have the RF 100-500 and love it. But what I've found using it on the 45mp R5, is that to get good sharpness when hand holding it, I need to shoot at 1/(4xfl), meaning 1/2000 sec if shooting at 500mm.
This might be different if you use a lower-mp body with it, I haven't tried since I do not have one, but I'd assume the bigger pixels would be more tolerant of micro movements.
Some examples here - most of the images, even panos, taken with the 100-500:
Both the below puffin shots were handheld and have been cropped a lot.
The first puffin was relatively stationary but the second one in flight is comparably slow showing wing movement but the eye is more than acceptably sharp for me (eye-af is amazing when panning). Puffin wings move up to 400 beat/minute. Maybe I was lucky but so far - so good!
I checked and most of my shots on my Flickr page using the RF100-500 are approximately 1/focal length and cropped (sometimes heavily).