It is difficult because there are a lot of minor details that has been added to Chez Scheme or to the patched version of Chez Scheme that is used by Racket.
For example ephemerons, that need some magic to cooperate with the garbage collector. https://cisco.github.io/ChezScheme/csug9.5/smgmt.html#./smgm...
[1] https://www.gnu.org/software/guile/
[2] https://www.gnu.org/software/guile/news/gnu-guile-300-releas...