Emacs API is just insane with too many special cases or with other packages unexpectedly affecting semantics. Even after stepping in debugger I often could not be sure if the things worked like I thought they should work with too much unstructured global state that is randomly poked.

I use Emacs since about five years straight, every day, and have used it on and off before, and have quite some Elisp in my .emacs.d, and can say that you're totally wrong on this. Yes the language has problems, the API, having been there since many decades ago, has quirks, but not more than most of the common languages out there, JS, C/C++, Python, Ruby, Perl, what not. What modifies the global state (mark, match, etc.) is well defined, and you can avoid learning it unless you're sending patches or maintaining your own packages. And dealing with programming languages' idiosyncacies is our raison d'etre anyways.

That doesn't quite ring any bell.. Have you used edebug? I haven't seen that level of code introspection and debbugability in any other tool, even a development environment.

Emacs allows you to look into and debug any function or variable, be it internal that shipped with its core, or one that you wrote.