After upgrading, I first only installed from the "standard" repos.
I didn't notice any problems with those apps (but didn't test long enough to be sure). All of my OVI store apps seemed to work fine.

The problems started (as you'd expect) when I re-installed apps from the 3 extras repos.
Two apps which had problems for me were the status menu plug-ins:
Advanced Power Monitor
Advanced Interface Switcher
(The apps froze the status menu immediately after installing. After a reboot, neither shows up in the status menu, and the battery level indicator no longer showed up in the status bar).

I've twice had to reflash already because of some conflicting app that shuts the device down and then stops it from booting again properly.
(But I haven't been able to work out yet what is causing the problem).

I'm now going to do a complete reflash (including eMMC) and test more carefully what the cause of the problems are after that.

For the sake of those that are nervous about upgrading, maybe it would be worth making special mention of the apps from the "regular/approved" sources that are not working (or are working) - and distinguishing which repository non-working apps come from.
If the various extras repos are enabled then seems quite unreasonable to expect all the apps to be working safely as soon as an OS update is released.