The problem with the Scheme standards is that the committee refuses to be opinionated about implementation details, and so the standard is defined in terms of itself with little to no consideration for the environment in which Scheme will operate. In practical terms, this means that if you want to communicate with other libraries, or virtually any aspect of the system on which you're running, then you're venturing outside of the Scheme specification and into implementation-specific territory.

Scheme is a toy language. The implementations are not, but then they aren't Scheme so much as they are Scheme with useful extensions.