What's left to be done?
Having said that it seems to be in a POC state, but is very slow and needs optimization. These two wiki pages provide a decent overview: https://www.emacswiki.org/emacs/GuileEmacs & https://www.emacswiki.org/emacs/GuileEmacsTodo
This "Preview: portable dumper," thread from November 2016 and spin-offs appear to be the most up-to-date emacs-devel discussion on the subject: https://lists.gnu.org/archive/html/emacs-devel/2016-11/threa...
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...
All the work on the Emacs integration appears to have been done by Robin Templeton in early 2015 with no commit since May 2015: http://git.hcoop.net/?p=bpt/emacs.git;a=shortlog
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.
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.