The risk is fairly low for updating maps
The worst thing that can happen is you loose your current map that is stored on the hard drive. This would happen if the DVD drive struggles to read all the info on the new map DVD. Some drives can be fussy what DVD brand they like.
As most disk that are available are copies from either ebay, friends or other forum members it is likely that some people will have issues.
I have a brand new unit at the minute that I have tried loads of copied dvds in and it fails to load the v9 maps, but as soon as I put the original V6 disk in it loads first time