Practically any UI change, for example, feels difficult at first
The Ribbon on MS products feels difficult because it IS difficult. Instead of a menu where you're scanning a bunch of equally sized elements with text, you're scanning a bunch of unevenly sized buttons with text and icons (which often are irrelevant or misleading). In addition, nesting options on the ribbon are often signaled in conflicting ways, and the overall grouping us much more broad, and therefor less sensible than with regular menus. With enough practice you can work around it, but even after years with it, I've never found it easier to use than traditional menus.
By contrast, another fairly major UI change in the MS space is the restructured start menu and task bar in Windows 7, and in that case almost everybody who's tried it has been won over immediately. They combined concepts (running apps and pinned launchers), removed data, shuffled some things around, and made it MORE clear than it used to be.
I don't buy that a UI change necessarily has to be jarring. Yes, the bar is higher to make the concepts more clear than they used to be, but if it's done right it can be an intuitive change at the same time as an improvement.