I've had a few issues with clean and upgrade. Although minimal issues with the clean install, most of mine are with upgrades.
Husbands Computer - Upgraded: 1) Playback devices randomly takes upwards of 30 seconds to open for some reason. Lucky, because he uses this a lot to switch between speakers and headphones.
2) Explorer randomly just won't open. This means whenever you try to open any type of folder it just refuses to actually open. Must either restart explorer or sometimes computer
3) Can't for the life of us get 3 monitors to output at once
My laptop - Upgraded: 1) Sluggish
2) Crashing was fairly common. Ctl Alt Del was never working when I needed it
3) Wifi would never connect on startup. With every boot I'd have to right click the wireless and click "troubleshoot problems" to get it to go again
4) Start menu sometimes didn't want to open, and every time it did open it was always with a delay.
-Probably a couple more things I don't remember
Same Laptop - Fresh Install + Install SSD: 1) Not sluggish anymore
2) Start menu still has a slight delay
3) Some random crashes, not near as often though
My Desktop - Upgraded: 1) Upgrades likes to fail, for some reason this one seems to upgrade at stupid times which causes this.
2) Some random crashes, but not terribly often
I'm afraid to fresh install this one, it came with Windows 8 and the activation seems to be very weird when I tried to clean install. Ended up have to ghost the original drive when I installed my SSD. I guess I'm old fashioned, I'm used to just installing and putting in my product key again.
Work Computer - Upgraded: 1) Sluggish
2) Windows Live Mail likes to just sit there at the splash screen sometimes. Need to reboot computer to get it to work again
3) Google Chrome like to not open sometimes, but not near as bad as Mail.
I'm amazed at how much it affects each computer differently. I know I'm forgetting tons of random stuff that's happened to these computers, but I've not had very good luck with the upgrade. Doing a clean install definitely helps, but it's still not as stable as Windows 7 and 8 in my experience.