It's my favorite Scheme. It's minimalist, but practical, and usable for Real Work. It's everything I like about Racket, without all thinks I don't like about Racket (Racket's a fine language, it's just not my thing).
It is not practical enough for me to have a language which does not support native threads.
Only Guile does that, AFAIK. And Chicken's threads are perfectly usable in most cases. And you can always fork(2), if you really want to.
And as I read it sometime ago while searching for Scheme implementations with native threads, Cheney on the MTA is inherently single threaded.
Not necessarily, it can be extended to support multithreading: