Dave, the magnifying function of EVF of mirrorless cameras (and of LCD screens of any digital cameras) are far more helpful to nailing the focus.
I'm concerned with the magnification of the complete frame not zooming in to a portion of the frame. You can't for example follow a bird in flight while zoomed in to 100% or even 50%, not in any practical sense. If the subject is static, say a bird sleeping on a branch you can zoom in focus and zoom out but not one in flight.
The Z50, a DX format, has a viewfinder magnification of 1.02x with a 50mm lens focused to infinity while the Z6 II, an FX format, has a viewfinder magnification with a 50mm lens focused to infinity of 0.80x. From this the advantage of the DX over cropping the FX might be the same as an optical viewfinder but if shooting with say a Z7 II is the frame shown with a grayed frame like a D850 might show or does the finder zoom in to show the full DX format. If the latter then a Z7 II in DX crop should be roughly on par with a Z50. As I say, I've never touched a mirroless Nikon. I'm sure a Z7 or Z7 II owner can tell me.
Dave
Now I find zooming to 100% with a D800, hand held almost, not quite useless. I did this with a 20/2.8 AIS in low light. I focused in live view then returned to the optical viewfinder to frame. I did my best to keep the lens to subject distance static. A tripod is surly the way to go if, and that's important, if a tripod is permitted. I think I stopped down to f/8 for DoF which helped.
I've messed around with focus peeking and found it quite useless for focusing a 50/1.2 AIS even stopped down to f/2.0~2.8.
Again I have no experience but I read or view that mirrorless viewfinders still have a lag that make them somewhat unsuitable for sports and wild life. I've got to find out why this computer corrects my spelling on the fly using the correctly spelled, damned wrong word!