Yeah. It's one of the most actively maintained Schemes. People just
don't know about it.
For a long time neither the head nor the latest release would even build, and I think people gave up on looking at it.
That's what I like about Scheme. Any environment or deployment
scenario you could possibly need to target, there's already an
implementation for the job.
QFT.
It'd be nice to have more portable FFI.
FFI in any form will not be part of R7RS-large by chair fiat; he committee voted it down in the earliest days, and I am committed not to bringing it back. There are just too many differences, even apart from Schemes running on JVM or CLR.
Note that r6rs-pffi is just a wrapper over the FFIs of some R6RS systems. In particular it does not support Ypsilon, which AFAIK has no native FFI. (Not surprising it can't do anything with Biwa or Iron, and Loko is probably too new, though it looks cool: it can run on bare metal as well as Linux: the home page is
http://scheme.fail.