In my eyes, the main problem with hand holding the RB is not it's size and weight. It is a stable camera when used against the chest with the WLF, and it is even OK at eye level. The mirror sounds intimidating, but it is actually very well damped. But you have to use it in pretty strong light to get the shutter speeds you need to hand hold it, and long lenses are pretty much out for this purpose. Focusing it quickly and accurately is also more difficult than with many cameras. It can be challenging to nail focus when shooting wide open.
I do shoot my RZ hand held quite a bit. I use the 110 f/2.8 and 65mm f/4 for this. I have not been that successful with the 210mm lens. (I did not expect to be, but I had to try!)
One good thing about the large frame is that you can use fast films and they won't appear very grainy or soft when you enlarge them. I use T-Max 400, Tri-X, and Fuji NPH more than any other films when shooting my RZ hand held.