One big item is that that Emacs' internal string/buffer representation supports things like loading weirdly (or even erroneously) encoded data, and saving this

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.