See also Sony A7R: Lens Support and Lens Mount Torque with Adapted Lenses (Sag, Warp Potential, Shutter Vibration), and Old Geezers Need an EVF: the Rear LCD and Presbyopia are a Bad Combination For Aging Eyes.
While there can be support concerns with large and heavy lenses and shutter vibration issues with the Sony A7R, one thing stands out: the EVF at full magnification on the Sony A7R (or A7) lets me nail the focus like no other system has ever done before.
Together with the long-throw roller bearing focusing (cine lens style) and mass of the Zeiss Otus 55/1.4 APO-Distagon and its best-lens-ever made optical performance, the combination deliver the sharpest images I can reliably make handheld—the optical viewfinder on the latest Nikon and Canon cameras is godawful for manual focus.
An EVF is essential if you want the world-class optical results for your subject. Hence what the #$*#$*#$* are Nikon and Canon doing at least not offering an EVF option and with Nikon having its mangled Live View. Enough already. It’s not that folks want the Sony A7R so much as that Nikon and Canon have done zero innovation for three years.
The downside? Closer range shooting can mean body movement of yourself or a person in-between focusing and shooting (you’re at a highly magnified view, so you have to press the button lightly, compose and shoot, and by then the distance might have changed—5mm matters).