Using this reducer since 6 months on an Esprit 120ED with a Full Frame camera: Nikon D750.
I don’t have any issues with sharpness in the corners however it depends on image to image. I suspect it’s related to the different rings to be thightened in the appropropriate manner (adaption rings).
See this image from M45 as a bad example with some elongated stars in the upper left and right corners.
See this image from B33 as a rather good example.
These are the masterflats that show the vignetting, one regular and one stretched.
Hoofd Platte gemaakt van 30 afbeeldingen (Rechtlijnig Kappa-Sigma (Kappa = 0.50, Herhaling = 20)) 
Hoofd Platte gemaakt van 30 afbeeldingen (Rechtlijnig Kappa-Sigma (Kappa = 0.50, Herhaling = 20))
This is a document from Teleskop-Service describing elongated stars with reducers/correctors (proudly stolen):