The one I have if you fold it all the way up so you can use the full length of the truck bed the view is blocked . It has a mid fold position that doesn't block anything in terms of view.
I have driven a super duty with a cargo shell on the back that has zero rear view visibility so I am use to that. And pulled box trailers that had no view from the center mirror. So I am use to it. Plus the only time I would have it all the way open would be when carrying some large cargo item that would obstruct the rear view.
If you are not comfortable with The rear window being blocked you could just remove the tri-fold cover entirely it's just two clamps to take it off. But again anything so big you would need the full bed space open would also probably block your view.
Mostly I will have it fully open just for loading and then fully closed for driving.
There are other trifold options that never block your view.
This is the first bed cover I have owned.