IIRC their main issues were threading instability in CMUCL on BSD or mac, which I believe has since been fixed in SBCL. In general, SBCL has come a long way, partly due to companies that use CL (e.g. ITA) funding its development. CL is in many ways much more production-ready than it used to be for that reason. It still has rough spots, but my experience is vastly improved over what it was 1.5 years ago.