What angle limits should apply to Graham condensers?
I have no idea why there are so many photos showing Graham condensers in a non-vertical position.  Maybe just because they look so cool and photos tend to be landscape.  The only time that might be safe is if there were a vacuum pulling to the collection side.

Graham condensers should be used vertically, precisely for the reasons you stated: Particularly because otherwise they will bubble, bump, and potentially explode.