If you read my post above, you'll see that I have occasionally encountered faulty/poorly-constructed swivels on the Domke Gripper straps. The joint is a simple ball & socket design & if the "ball" is too small to begin with, it can easily wear down in size to the point where it will pull right through the socket. A properly-constructed Gripper Swivel is easily strong enough to hold a Rolleiflex, even w/a prism finder. I would recommend checking the strap in the store before buying [edit: I see you're in DC, so I would recommend Penn on E St. or Pro Photo on I St., as they have a pretty good selection], or inspecting it closely upon receipt if buying online, & reject any that have "balls" that are too small (no puns intended 😉 ).
Because I tend to use my straps (& cameras) pretty hard & for ergonomic comfort, I have switched to Op/Tech for my heavier cameras/setups, like the Norita 66, as they lack any joint & have the soft, stretchy shoulder pads. For a Rolleiflex, however, it will depend on whether I'm using the prism finder; if so, I'll often use the Op/Tech to be on the safe side, if not, the Gripper (but I always check the condition of the swivel joint).
If you use the non-grippy side of the pad, the Grippers do slide pretty easily, the same as any fabric strap.