One of the things I found with my "viewable" racks is that I was sliding the boxes open less often. When I needed to check for dirty water bowls, uneaten rodents, or even eggs I could simply look through the front of the box. My best guess is that the stress put on the snake by frequent opening of the boxes outweights that of the brighter light from the more clear material.
For species that don't use hide boxes perhaps there is some stress from people walking by, etc.
I used to keep a lot of terrestrial vipers. Most of them would not use hide boxes nor burrow in their substrate. In a clear fronted cage some specimens would strike the glass/poly/acrylic when I walked by. This did not seem to be a problem in the opaque rubbermaids and sterilites.
With cobras, on the other hand, they would strike when I walked by no matter what the cage they were in.
In that regard I suspect that a cage with viewing might be more stressful for the inhabitants.