If it's actually standard compliant any library written in a RxRS-compliant Scheme dialect would be available. The FFI seems reasonably sane as well. They list

If it's actually standard compliant any library written in a RxRS-compliant Scheme dialect would be available. The FFI seems reasonably sane as well. They list a lot of libraries in the reference but some are lacking documentation. At a glance it seems you could do a lot in systems and network programming, but if you want to produce a lot of PDF/A or run a cryptocoin business maybe use something else?