There are no frame lines in the external optical viewfinder, so the visible limits of the frame depend largely on the position of the eye.
For example, this is the view through the viewfinder mounted on the Sony a7c with the Pentax K 35/3.5 lens:
You can see parts of the lens and the lens adapter. This is because the shoe is quite close to the lens, as the Sony a7c does not have a large SLR-style top. But this reduces the vertical parallax.
And a picture taken:
Overall, the viewfinder is usable. And as I said, the image quality is surprisingly good. Let's hope it doesn't deteriorate over time.