Chez scheme is still around and is a pretty stellar piece of software. I have found that finding bugs in smaller language implementations is uncommon, but not rare. At least once you leave the happy path.
I have written well over 100k lines of scheme that runs on chez and I have had zero issues. The documentation is clear with good descriptions of even hairy topics like macro phasing and the module system.
I know what Chez Scheme is, but I had not heard of this FFP machine before.
https://stackoverflow.com/questions/2147821/what-is-an-ffp-m...
Racket has replaced their C codebase with Chez Scheme, yet another success story.
Incredibly, they support OS X on PowerPC.