The thing is that you don't really learn Emacs. Emacs is just an interface to programs written in Elisp that happens to have editor-like functionality out of the box. There are specific conventions it follows, and Elisp has its own set of quirks and a steep learning curve if the user is not fluent in other Lisp languages, so those can be challenging to learn. But boiled down to its core, there's not much to Emacs itself. It's actually a pretty bad editor OOB, but that might be subjective.

What is more challenging is actually customizing Emacs to suit your own preferences, which is the main reason you would want to use it. Deciding which external packages to install, how to use and customize them, and writing your own packages. That is a lifelong endeavor and learning experience, just like programming. So it's to be expected that someone might be a long-time Emacs user, but not be aware of some of its features. You're not expected to know everything, nor do you need to. Just learn as much as you need to make it useful for you. Essentially, every Emacs installation is different, since it's purpose built for a specific use case according to very specific preferences. There's no singular "Emacs". And that is a beautiful thing. :)

I was really talking about the builtin libraries (listed in the core manual). That in itself is a challenge. The surface is huge. Not a critic per se, I've been using emacs for 25 years, I'm just reporting.

Emacs is a path, not a destination!

Saying this as somebody who's been coevolving with emacs for 20 years already. Oh, all the little bits of lisp, all the mechanical memory, all the IDEs that came and went...

My emacs survived them all!

Generally speaking, unless you're doing something weird (typeset IEC ladder!), proprietary (code completion for Aveva Quickscipt!), or right on the cutting edge, someone has figured out a way of doing what you need in Emacs. You just have to identify the pain points first. That's harder than people think.