I run both KDE and GNOME (plus XFCE, Fluxbox and LXDE). GNOME can cause
problems with other desktops. For example, if you run Nautilus in KDE or
XFCE then you will lose your desktop. The trick is to run nautilus with the
--no-desktop switch. The command looks like this nautilus --no-desktop. This
problem is not permanent, Once nautilus is shut down then you get your
desktop back.

There are other problems, but most are not serious. I find it useful to have
more than one desktop environment because if one fails then you can fix it
from the other. Most KDE problems can be fixed by deleting the .kde folder
which gives you with a fresh beginning. However, if you have a partial
upgrade then you may not be able to get into a desktop environment until the
upgrade is completed. This also means that it is good to have a second
environment.