I've been using computers since Apple IIe and am much happier with things today than versions of the past. There are certainly hiccups along the way, but no way am I going to trade what I have today for what I used in 80s/90s.
You can customize every section and it's very easy to pick up, I'm sure if you'd have to spend some time with office nowadays you'd agree. I'd take them over old-style nondescript toolbars by now.
To me the ribbon was almost peak UI. It's tainted because it was introduced around the same time that designers started winning the "user friendly" vs "looks pretty" battle, but the ribbon is definitely from the "user friendly" side. They provide large targets, they have icons and text to help remember them better, the button size varies according to relative likelihood, some menu is always open instead of closing as soon as you start doing something. The ribbon was a genuine attempt at giving all users access to a powerful piece of software: a tool to upgrade the mental model of users instead of reaching for the lowest common denominator.
The biggest limitation of them seems to be that they don't help teach people the shortcut keys. To me that's not important because I'm never going to use them, but others like them, and by hiding them they've gone from seeming like a utility for experienced users to being a secret for hackers that could be removed at any time.
But they seem like a genuine improvement in contrast to the old system of either text-or-icons, absence of meaningful hierarchy, menus that are hard to study because as soon as you interact with them they disappear, or the new system of gratuitously hidden options, secret buttons, and apps which prioritise branding and a sense of eternal instability and change over predictability and approachability.
If I have to believe this[1] thread, then Office 365 is now an Electron app, which would explain the slowness.
Sadly, support ended October of 2020, so it's time to find something else before too long. It is not going to be contemporary Office.