It's such an amazing project, I wish it used something other than Guile but you can't have everything.
A few problems remain though. A good debugger, a good macro expander (geiser in Emacs is able to expand somehow), and solving the issues with R6RS library syntax and standard library bindings, are what comes to mind.
Racket's multi-core abilities for a long time were mostly heavy weight (places, starting whole new Racket VMs), except for their implementation of futures, but that one was not always useful. I think recently the situation in Racket has improved, but I don't know, whether it is as good as Guile fibers and futures (which are different from futures in Racket).