Well, I'm sure you'd concede that read-file-name is part of Emacs's core, and its been buggy for years. (Have not checked to see yet if it is still buggy in version 25.) Specifically, if the current buffer is visiting an image file, pressing down arrow during a read-file-name will often cause read-file-name to try to insert the whole image as a text string into the minibuffer after the prompt, which will make Emacs unresponsive for a few seconds and will sometimes crash Emacs.

Before that there were years during which there was no way to configure Emacs that I was able to find to refrain from asking the user what coding system he wants to use every time a buffer containing a non-ascii character is saved. Not strictly a bug since maybe there are users who do not mind regularly getting into the same dialog with their text editor, but certainly, so many occurrences of the same monotonous dialog is unusual behavior in a user-facing application in this decade and very tedious at least to this particular user.

One reason Emacs is difficult to debug is because of the widespread use of global variables.

That said, I haven't found anything worth leaving Emacs for. (If I were a web dev, maybe I would've left it for vscode by now, but I don't like how text looks in vscode: I prefer the native OS X text rendering used by almost all text editors except the Electron-based ones.)