The complexity can leave you with weird bugs. For example if you use regular emacs (X forwarding over a network) on X displays with different dpis, your text will look the same (physical) size on both. If you use spacemacs it can be wrong on one because of some hack someone added somewhere in that tower of complexity. There are plenty of other oddities in there.
Also as soon as one advances to wanting slightly more customisation, it can be really hard to do it with spacemacs. Whereas with regular emacs it’s quite easy to go from basically zero to some.
Spacemacs made my transition from Vim much smoother, and introduced me to many great packages and ways of configuring Emacs, so I'll always be grateful for that. It's a wonderful project.
But after a few months I started noticing those issues it introduced because of the additional layers of complexity, and decided to start from a scratch .emacs.d and add packages I strictly needed. It's surprising how much can be accomplished with just use-package and a init.d inspired loading structure.
The best thing about Emacs is its malleability, and relying on others to do this for you can be helpful, but is usually not what works best. I suppose this also applies to every 3rd party package you depend on, but I'm not versed in Elisp enough yet to _really_ build my Emacs experience from scratch. ;)
And just like in the world of Lisp, after you add a new functionality, it becomes indistinguishable from what you already have. That is, it looks like it has been there all along, from the start. It doesn't look like a 3rd party plugin that you need to take special precautions to install. It just blends in nicely, and naturally.
Or, to put it another way, if you learn (Emacs) Lisp and you start to tweak Emacs to your own liking, you can transform Emacs into your own personal tool, adjusted exactly the way you like it, no questions asked.
"Consistency" is a good word. The result, for me at least, is that I can actually think about what I want to do with the tool and not how to use it. It gets out of the way, it does not try to fight me.
I think maybe that's one interpretation of the saying "Emacs is an operating system".
Emacs is also an interactive IDE for elisp development. It provides all of the features you'd ever want from an IDE and then some. You get a linter, a debugger, semantic completion, ability to execute whole files or parts of them, data inspector/visualizer, finding definitions and references to symbols, a list of functions in a given file and across many files, shortcuts for common editing tasks, refactoring support, and so on. It's a very featureful, modern IDE with special emphasis on interactivity.
The effect of these two characteristics is that in Emacs you can easily find an implementation of any feature (no matter if it comes from Emacs core or some plugin), then edit it and see the results instantly (without the need for as much as saving the file), all the while having access to all of the IDE-like tools. You can customize every part of Emacs to exactly fit your needs - if there's something you'd like to change, but the feature's author didn't create a setting option for it, you can go locate and change the implementation in a few minutes.
When a computer game comes out, sometimes the authors also release a development kit with things like a map editor, theming support, tweaking AI and so on. Using (most) other editors is like simply playing the game: you can get better at it, but you can't change the rules. Using Emacs is like playing the game with the ability to instantly switch to the dev kit: you can simply play the game, but you also can bend, change, or rewrite the rules as you wish. With (most) other editors you're a consumer; with Emacs, you are the owner.
To really make use of all of that you have to know elisp, which is why knowing it is important. Of course, you can happily Emacs without dabbling in its configuration too much, but in that case, Emacs becomes just another editor with lots of awkward key bindings and a huge amount of cruft accreted during the last 40+ years.