When in full screen, and with rulers on, if you go to the very edge of the screen to grab a ruler, the cursor will turn to a resize arrow instead and grab the window, only for it to snap back into place (because it's full screen).
I'm running Yosemite. Rulers in either Full Screen mode or FS mode with menu bar, are working fine. I can easily drag a guide off them. However, I don't have a Retina or HiDPI monitor on this computer. Perhaps that makes it more difficult, easier to move the cursor slightly past the ruler?