I've done it for a leg vice. It's definitely on the rattley side, and the jaw can move around a few millimeters when not loaded. Once you get some tension on it, it tends to stay in place, but if I want to clamp something in a way that the vice jaw is flush with the workbench top, it can take a couple of tries to set it just right. As a coarse, just get it done solution, it's fine. No speed nut.
The screw itself is huge and seems like that should make it really strong, but I think the reality is that even a 1/2" screw is stronger than your puny human arms, the diameter of the scaffold screw means you get less mechanical advantage from the handle, and the high pitch of the thread further reduces you mechanical advantage.