The Ribbon is a disaster. Compared to conventional toolbars, it fails across several metrics.

When it first came out, I did studies of myself using it vs. the older toolbared versions of Word and Excel, and found I was quantifiably slower. This was after spending enough time to familiarize myself with it and get over any learning curve.

EFFICIENCY

The biggest problem is it introduced more clicks to get things done - in some cases twice as many or more. Having to "tab" to the correct ribbon pane introduces an extra click for every task that used to be one click away, unless the button happens to be on the same tab. Unfortunately the grouping wasn't as well thought out as it could have been. It was designed with a strong bias for "discoverability" over efficiency, and I found with many repetitive tasks that I commonly carried out, I was constantly having to switch back and forth between tabs. That doesn't even get into the extra clicks required for fancier elements like dropdowns, etc. And certain panes they couldn't figure out where to put are clearly "bolted" on.

KEYBOARD SHORTCUTS

At the same time, Microsoft de-emphasized keyboard accelerators. So where the old toolbar used to hint you the keyboard shortcut in a tooltip every time you rested your mouse over a button, the new one doesn't - making it unlikely users will ever learn the powerful key combos that enable more rapid interaction and reduce RSI caused by mousing (repetitive strain injury). In my case this manifests as physical pain, so I'm very aware of wasteful gestures.

SCREEN REAL ESTATE

The amount of text in the button captions on the ribbon is also excessive. It really isn't a toolbar at all, more of a fancy dropdown menu that's been pivoted horizontally instead of vertical. It turned the menu bar, which used to be a nice, compact, single line, into something that now takes up ~4x as much vertical screen real estate. As most users' monitors are in landscape orientation, vertical space is scare to start with; congratulations you just wasted more of those precious pixels, robbing me of space to look at what I really care about which is the document or whatever thing I'm actually working on.

DISCOVERABILITY

You used to be able to get a good sense of most software's major functionality by strolling through all the menu options. Mastery (or at least proficiency) was straightforward. With the more dynamic paradigm Microsoft adopted along with the Ribbon, there's lots of functionality you don't even see until you're in a new situation (or that's hidden to the responsive window layout, which is ironic - instead of making the thing more compact, they made portions of it disappear if your window is too small). I grant some may argue this has benefits for not appearing as overwhelming to new users (although personally I've always found clean, uniform, well thought out menus to be less jarring than the scattered and more artistically inclined ribbon). But easing the learning curve had the trade off of making those users perceptually stuck in "beginner" mode. They can't customize the ribbon as meaningfully (I used to always tailor the toolbar by removing all the icons I already knew the keyboard shortcuts for, adding some buttons that were missing like Strikethrough, and move it to the same row as the menu bar to maximize clientarea space)

In my case, after trying out the new versions for a year, I made an intentional decision to go back to the 2003 versions of Word and Excel, and never look back (forward?). They are my daily drivers. These days, I barely touch modern versions of Word and Excel, except for the very rare instance I actually need a specific new feature (i.e. a spreadsheet with more than 65k rows). If someone asks me to use the new version, I simply refuse (which has never been a showstopper - my work quality is preeminent, and once you get past policy bureaucracy it turns out clients/employers don't care what tool I use to get it done).

The whole point of a toolbar was always to be a place you could pin commands you want instant access to, just a click away. The ribbon shredded that paradigm, and in my opinion took us a marked step backward in computing. It fails across several metrics, compared to regular toolbars. I wanted to blog about it at the time in hopes of convincing the world it was a mistake, but didn't have the free time. 20 years later, I'm curious if more people share these sentiments and acknowledge its shortcomings.