- performance is not great (although it’s getting better with the advent of native compilation in Emacs 28)
- the system language is awkward (no namespaces, dynamic scoping by default, ...)
- no preemptive multithreading
- etc.
On the plus side, it is auto-documenting and extremely malleable.
[0]: Yeah, it can be a full-fledged OS. Just add a Linux kernel: http://informatimago.free.fr/i/linux/emacs-on-user-mode-linu...
RMS's dislike of Common Lisp really caused a colossal amount of damage. Imagine an Emacs written in Common Lisp instead of Emacs Lisp, with decades and decades of improvements. Having written a reasonable amount of software in both, Common Lisp Emacs would be very, very preferable, in part for the reasons you list and in part for others (e.g. a full-fledged, built in object system and improved extensibility).
If Emacs were written in Common Lisp, only people on big mainframes would have been able to use it in 1985, and it probably wouldn't have become popular in the first place. Also, implementing Common Lisp would have taken much longer. RMS has said that his main goal with his LISP dialect was to keep the programming language as small and performant as possible, since at that time, machines had maybe 1MB of RAM (if you were lucky) and no virtual memory. This is why for instance he decided against lexical scoping (and still, Emacs was known back then as "eight megabytes and constantly swapping").
[0]: https://github.com/lokke-org/lokke [1]: https://gitlab.com/python-on-guile/python-on-guile
[1] https://nyxt.atlas.engineer/article/why-building-nyxt-instea...
This is not much of a problem in practice these days. As you probably know it's trivial to enable lexical-binding for an entire source file and many (most?) modern packages do just that. It's even on by default in the scratch buffer. Emacs is just more user friendly than most other systems when when it comes to backwards compatibility.
Long line performance is still a huge problem. Emacs team does an amazing job keeping up, but the age shows itself everywhere. In the end I've realized that org/magit were not life-changing for me, evil-mode it the only thing I care about. :)
I don't even use LSP because I have my own ways of browsing source code through emacs. So I go off and tout emacs' amazingness, but then when someone tries it and attempts to use what worked in other editors, it's a slow laggy mess.
I don't think people should try to use Emacs to build an OS on top of, but building an OS without understanding why Emacs survived is going to just end us up in the same world of inflexible, brittle, clunky software we're in now.