The short answer is “yes”. The long answer is, well… long.
Some of the vignetting is of course an unavoidable product of physics as described by Isaac Newton in his "inverse square law" – light loses intensity the further it travels. The distance from the lens to the center of the film plane is shorter than it is at the edges, so there is always some loss of light at the edges when compared to the center. This effect becomes more noticeable as the distances become greater in large format photography. Nothing can be done about the physics. It is indeed once of the defining characteristics of large format photography.
Some of the effect is caused because light rays travel in a straight line from the subject, through the lens aperture, to the film plane. If the light is exposing a negative at the film plane, that doesn't really matter because all the light stops there and is absorbed. But if you allow the light to pass through that point – as with a ground glass or our focussing screen – then the rays continue to spread out and get lost. This effect is more prominent in short focal length lenses as they must be positioned closer to the film plane.
Little can be done to counter this effect, the complex and expensive use of fresnel lenses that linearize the rays can be introduced, but these present significant mechanical and optical challenges and work best at one specific focal length.
Fresnel lenses would need to be placed between the lens and the focus plane, and again between the focal plane and the objective camera, which would be an entirely different invention than the ObscuraFlex™. And in the case of a Graflex this would compromise the camera for normal photography, defeating the primary purpose of the ObscuraFlex™ as a tool to assist large format film photography while leaving the camera untouched.
ObscuraFlex™ uses a solution that reduces the vignette effect when compared to a classic ground glass, but cannot eliminate it.
For clear example images that show the difference, see: "Is it Just Shooting the Ground Glass?" FAQ
The reality is, there’s a direct trade-off between clarity, brightness, and the reduction of the vignette. The more light that gets stopped at the focusing screen, the more even the image appears. But you rapidly reach a point where so much light gets stopped that not enough passes through to be visible, and the act of “catching” the light is essentially diffusion, which by its nature degrades image sharpness. So it’s a balancing act.
Is the resulting image as good as the clarity and detail of an expertly exposed 4x5 negative?
Definitely not.
Is it good enough for social media, or to print as a 8x10 and put on the wall, or to teach the principles of large format photography? Absolutely. Is the resulting image on your phone screen a useful tool for focus and composition when shooting film that also shoots reference stills, video, or as many test shots as you like without waiting for a lab or paying for film? Yup.