I have used the Inon viewfinder for many years. Then Seacam and now the Nauticam viewfinder for 8 years. Optically, the Inon viewfinder cannot keep up. It is much darker and has a narrower field of vision. On the other hand, there is almost $1000 in between. Nevertheless, as with lenses, money is well spent on good glass.
Dave's tip with the viewfinder reduction in the Nikon Z8 is really great.