1. Latency. When you click a dropdown, the whole interface can freeze for several seconds while Word presumably renders the menu or something. I can just about understand that for the fonts menu, but for the "bullets and enumerations" dropdown it's just plain bad.
2. Things moving around in the ribbon because Microsoft decided to push an auto-update to include some new search with bing or "data insights" feature.
The number of times I had to use "Tell me what you want to do"-field is on a stupid level compared to the rich toolbar and extensive menus that existed pre-2007.
So why in the heck the ribbon even needs to exist is baffling - just make a command search/command line input, and then if it's literally on a toolbar on screen, highlight it for me (configurably) so I as a new user know what to click on next time.
The ribbon is just somehow, always exactly wrong.
If you wanted to see how a font looked in the classic Word UI, you highlighted a text and selected a font from the dropdown. When the dropdown was changed to show the actual font, it was the first step towards the ribbon, where the UI generally shows the actual effect of the options.
Programmers does not understand this difference, since they are used to work on abstraction levels removed from the actual output. So the ribbon seem like a fad just like the mouse and icons was seen as a fad by many developers back in the day. ("Why don't people just learn LaTeX?")