I had NO PROBLEMS upgrading from W7 to W8. That worked fine. Problems were 'old' drivers that the Upgrade Advisor DID not identify. Same goes for programs it said would work. Same for the need to re-install some apps/drivers that were NOT identified as being needed.
My Network built-in RealTek NIC 'worked', but didn't connect all the time on boot. Found I had to disable it, and enable it, and it connected then. Discovered a W8 release of the driver that didn't work either. This was an older DELL, and DELL did NOT have W8 files for this model. Eventually tried a newer DELL model of this computer (XPS 435T, newer is XPS 8500, which my wife has) that had a RealTek W8 file. That file was older than the latest RealTek one but once installed works like a charm.
Norton Ghost 15 DOES NOT work, although the Advisor said it did... and I'm waiting for a fix/update or the need to buy SSR 2013 which was just released for W8 but costs almost twice as much... may switch back-up image programs, not sure yet.
Had to re-install my HP wireless printer apps and driver. Only on my computer, on my wife's it wasn't necessary.
A lot of 'bumps' along the way as you discover programs that don't work. First thing to try, re-install and/or check to make sure you have the latest. Wife has discovered MANY GAMEHOUSE games are not working and GAMEHOUSE may not update them either.
One thing, actually two, I'd do before doing the upgrade, run the ADD/REMOVE programs and TAKE OUT (DELETE/REMOVE) and app you DO NOT want. Also run DISK CLEANUP as well, and make sure you take out SYSTEM FILES too... less there the better off you will be. Upgrade makes a WINDOWS.OLD folder and it can be large, maybe 20GB's or more, so make SURE you have at least that much or more FREE SPACE on C:. After you've run the ADD/REMOVE I'd run CCLEANER and clean the apps that are left and the Registry. Then MAKE a back-up in case the UPGRADE should fail, then UPGRADE.