I’ve found that when I resize my window from a size that’s small enough to trigger the mobile menu, to a size large enough that the full menu appears, no sub-menus appear when I hover over the appropriate primary navigation item. (You can see the problem on my site with the “About” menu – http://www.escapesacramento.com/wp/. Please ignore the fact that none of the links actually work; I’ve changed them all in preparation to move my site from the /wp/ subfolder to the root.) I can see this being a problem for anyone who starts by viewing my web site in a smaller window, then maximizes it; they will not have access to any of the pages linked from the sub-menu.
To give credit where credit is due, this is really the only thing that’s keeping me from giving Sydney a 5-star review; I’m a big fan of it in all other respects, including responsive/mobile design, but I’m not looking forward to completely redesigning the menu options.