There are many technical, optical and other reasons why surgeons use direct viewing stereo microscopes for micro-surgery.
Much of this has to do with depth perception. It is nearly impossible to see and gauge depth on a flat 2D image. Don't believe this, try driving a motor vehicle with one eye closed and note the limited depth-distance perception while driving a motor vehicle.
The other skill that must be learned for using a stereo microscope properly is adjustment and alignment of the eye pieces to the eyes of the specific user. This is critical to reduction of eye fatigue, eye strain and reception of a high definition optical image. Stereo eye pieces must have separation distance adjustment and at least one eye piece with adjustable length to compensate for the reality that human eyes are never perfectly identical for visual balance. Other factors are optical alignment of the microscope which can have a extreme effect of user eye fatigue and overall user experience.
The other major factors in eye strain and fatigue when using a microscope is optical quality for resolution, color rendition, contrast and image stability. This is a direct matter of microscope quality. Some of the very best stereo microscopes made were from Wild Heerbrugg (Swiss) and Zeiss (West Germany). These offer some of the very best stereo microscope optics available. When they are in good optical condition, proper optical alignment, with the eye pieces set up properly for a specific user's eye's, there is little to absolutely no eye fatigue even after hours of use. When all is proper stereo microscopes of this caliber are an absolute pleasure to use and own.
Other factors are lighting. Quality of light, color temperature of the light source and method of lighting will can have as much of an effect on the microscope image as the microscope itself.
As for magnification, 3X to 10X is plenty for SMD work, resolution, contrast, color rendition and all those other factors are often far more important than basic magnification.
Bernice