To illustrate my point.
There is a discussion about monoculars at BPL. One , that obviously knows what he is talking about, recommended the Barska 10x40mm.
Another member just posted this comment :
"My Barska 10x40 monocular arrived yesterday, and I will be returning it.

The problem is that the view through the monocular is either in focus in the center of the lens and out of focus in the outside half, or vice versa. The focused area does not cover the entire field of view - only the center half.

Very disappointing."
Having sold binos for over 30 years, I can tell you that it happens a lot...

Franco