I find its discoverability is terrible. I am always hunting for what I want to do and it's never anywhere that seems to me to be sensible. I usually end up doing a google search for what I want. Perusing the ribbon takes me much more time than just looking at the various options under the old style menus.
Also traditional menus had some traditional standards. Once you learned what was under "File" or "View" or "Insert" or "Format" it was often pretty similar across applications.
Logically, users have to learn the name of the tool before performing any sort of geographical associations (which menu, symbol, etc to find the tool).
There is no faster discoverability than O(log(N)) search using the letters of the name as a lookup.
The biggest failure of modern operating systems is failing to standardize this innate reality.
Windows,Linux,etc should have 1. keyboard button to jump to search box 2. type 1-2 letters and hit enter. Operating systems and applications should all have this kind of interface.
The most ironic apps have a ribbon named something like "Edit" but then the most used "edit" command lives in an unrelated ribbon.
Also traditional menus had some traditional standards. Once you learned what was under "File" or "View" or "Insert" or "Format" it was often pretty similar across applications.