Does the balance suddenly change the longer you use it?
I tried it for long enough to know that it didn't balance well, even less so in portrait orientation (which is how I shoot a lot of my marathon pics). To give an example of the difference, I could (if for some bizarre reason I wanted to) use my D850 with 70-200mm f2.8 in one hand, not so with the A7riii as there's too much rotational force on the hand. Obviously I would never use it one handed (as much for the strain on the mount as anything) but it was just an example of the difference in balance