The older October 2015 "In support of guile-emacs" thread seems to have been written after development mostly tapered out, discusses some work to be done, and is a call for more development (which seems to have gone unanswered): https://lists.gnu.org/archive/html/emacs-devel/2015-10/threa...
> Thanks to the work of Robin Templeton, Guile's Elisp implementation is now fully Emacs-compatible, implementing all of Elisp's features and quirks in the same way as the editor we know and love.
Robin Templeton also seems to be the one which finished off the elisp compatibility on Guile here so it's probably because he saw that as important to get first.
One big item is that that Emacs' internal string/buffer representation supports things like loading weirdly (or even erroneously) encoded data, and saving this unchanged; this is sometimes quite useful.
Guile's strings (gnulib's) do not support that, and adding the support is quite a bit of works for all the corner-cases that emacs supports.
Now, emacs and guile do not necessarily have to use the same underlying string/buffer implementation, but it would definitely be a big plus, and I wouldn't be surprised the emacs maintainers would require it.
