30 years of cruft + mix/matching of semi-incompatible semi-backwards-compatible systems if you use 3rd-party elisp and/or maintain a config for a long time.

I love emacs, use it daily, and think that it's positives far outweigh it's negatives, but hooo boy sometimes you need to wade through decades of muck to make it do a thing, and it's often not clear what the best way to go about trying to do a thing is, particularly to people who haven't been using emacs for a decade+.

My biggest problem with Emacs is that most modes map keys to be compatible with its default layout, so you really can't get away from the default Keybindings.

Evil-mode is nice in this regard, but I don't want Vim's defaults either, not to mention Emacs' bindings are still there, just hiding in insert mode.

The feature I want the most (that isn't readily available in an editor) is to define keybindings from scratch. I don't want to deal with undoing everyone else's work just to get started on my own.

bad performance and lots of bugs.