Don't forget that the MF rangefinders are not as dinky as Leicas. Using one of those, you are holding s lot of camera up in front of your face.
Lots of camera that needs to be supported by your arms. Waist level cameras allow pressing the elbows against the sides of your body. Having to raise a camera to eye level makes that a bit more difficult (that's why 45 degree prisms are so good - the perfect compromise between handholdability and prism viewing), depending on where the release button is (usually on top) even impossible.
So not necessarily more stable.