Theoretician

There seems to be a lot more that's broken than just the App Data. When I rolled back via a System Restore it broke iCue even more. It stopped recognising all of my Corsair devices and wouldn't recognise them even after they'd been reset.

Agree with that entirely.

My experience of upgrading to 3.20, and rolling back to 3.18 again, was bad at both ends! Details in my post here.

My experience is somewhat similar - especially the failure to detect devices when iCue was rolled back. Here's my experience - in some detail in case any of this is useful to anyone... My devices: K95 RGB keyboard, Scimitar Pro RGB mouse, MM800 RGB Polaris mousemat. My PC specs are in my profile. 1. Updated to iCue 3.20.80 (previous version was 3.18). 2. It changed my screen resolution. Restart didn't fix, had to manually change resolution. 3. Games became unplayable. Frame rates dropped to 1 or 2 FPS, games sometimes hung forever on loading screens. 4. iCue ate up loads of CPU when in focus (c.30%). When not in focus, it went back to around 0.5%. 5. NVIDIA Graphics Driver and Geforce Experience both disappeared from the PC. I had to reinstall them, and NVIDIA's install process confirmed that both were missing... 6. Did a System Restore to just prior to the iCue 3.20 install. This took iCue back to 3.18 and made the system run smoother. 7. But... like the OP, iCue completely stopped detecting all my Corsair devices. 8. Unable to reinstall or reset the devices. Unplugging and replugging didn't work, nor did following Corsair's reset instructions. 9. Did another System Restore, reverting back to the iCue 3.20.80 update. Got all my Corsair devices and setting back (a bit scrambled but things were present and functional). 10. iCue seemed to be eating a bit more CPU than usual when in focus, but it wasn't a huge problem. When all three Corsair devices were plugged in, CPU usage spiked to about 7% but quickly levelled out at around 2.5%. 11. But... NVIDIA was gone again... and needed a reinstall (same as above). 12. Once NVIDIA was reinstalled, PC needed restarting to get the Start menu to work. 13. And another but... some Windows default programs stopped working (Photos, Store, Mail), and needed a manual repair. 14. So far, programs and games are functioning ok, although games are perhaps a bit slower to load and to play.

I also saw high CPU usage when I first updated to 3.20. I rolled back to 3.18 but that lost all my profiles and iCue stopped detecting my Corsair devices. So I rolled back to 3.20 again (more precisely 3.20.80). I got my profiles back, this time CPU usage seems somewhat better, but oddly (and perhaps unrelatedly) some of the Microsoft apps broke and needed repairing.