Gave building Benben a shot while waiting for some tests to finish and sadly, this looks too complicated for me right now.
1. The Crystal build tooling in #Nixpkgs (crystal2nix tool & crystal.buildCrystalPackage function) assumes that nothing but Git is used, while Benben & most of its dependencies use Fossil.
2. The Zstd Crystal binding that Yunosynth requires has scripts with shebangs that don't exist on NixOS / the Nix sandbox. Not sure how to get that patched in with the way the build process works (first time looking at this stuff).
3. Trying to bypass all of this with a FHS sandbox gets me up to the linking step, then it all comes crumbling down with linker errors.
Maybe another time. 
