One thing that concerns me actually with dark visors ... are they UV filtered?
Point is that if they are not, they are just dark, means your iris's will open the pupil to adjust to the lower light ... yet you are getting the same UV ... but your 'eyes' are actually open more ... this can't be good for your eyes/retina!!
We all know to buy UV sunglasses these days (although not many actually realise why
) .. but what about UV dark visors? ... I am a bit concerned by this myself ... I use dark 'tearoffs' as my visor darkener ... I'd be surprised if they were UV filtered!!
As for getting sunburn ... assuming you are wearing the appropriate gear, the only bits of skin exposed will be your face (through visor) and neck (back and sides mostly).