> compensates for a mish-mash of inefficient and insecure technology on the backend side of things. . . no matter how optimized and clean the back end is (relative to OS X)
I'm not sure that's really an accurate characterization of OSX. In terms of security, I don't see where there have been any great number of exploits attacking the kernel (XNU) or userland (mostly FreeBSD) layers. Recent high-profile vulnerabilities like heartbleed were hardly OSX specific. I'm also not sure that those layers can really be called inefficient relative to Linux. Mach was a great deal more modular in terms of separation of concerns as compared to Linux (good for cleanliness/security, but never had great performance). However, with their XNU kernel, they seem to have made some of the same tradeoffs as Linux with respect to pure microkernel separation of concerns.
OSX has also done, in my opinion, a good job of rethinking parts of the traditional Unix infrastructure that needed rethinking. Check out JKH's talk about FreeBSD and potential future directions, and see how much that is informed by his experience leading the Unix technologies group at Apple (for the record, JKH was also one of the cofounders of FreeBSD, so he's not exactly a slouch): http://youtu.be/Mri66Uz6-8Y
I'd say the biggest reasons OSX never took off as a server OS are 1) licensing 2) coupling to Apple hardware 3) most of Apple's value-add is for desktops, laptops, and smaller portables--in other words, not compellingly better than other free nixen on the server 4) other nixen focussed on integrating the Chef/Puppet/Docker/virtualization/etc tech that you mention makes life for sysadmins easier.