I've only had one time where an update(s) did some harm, that i had little control over. i think it was xorg-server 1.10/11 maybe, and there were some issues with my wacom and gesture recognition software. by the second update after breakage, it was fixed. i just stuck with the last working xorg-server, until it was worked again. (that was like 4-5 days later).
Other things that i commonly have to deal with on updates (which i don't consider breakage) is having to do things like recompile vmware modules, and re-install nvidia whenever i upgrade my kernel.
As far as KDE, i don't use it. too much bloat and stuff that i don't really need, so i can't really comment, in any depth, as to how well the KDE experience is in Archlinux. I use Compiz/Gnome3 (without GDM, gnome-panel and some other gnome components) on both my Arch installs. it seems to work well, and complexity is reduced by only using / running the components i need.