To be absolutely fair, the first adequate namespaces implementation came out in kernel version 3.8, so that puts us in 2013, when talking capabilities to run %process% in a "container". Admittedly, a few years later than illumos. But then again, illumos was just playing catch-up with *BSD back then, as jails were available since 2000 ;)
By the way, what is your primary motivation for using illumos? genuinely curious.
Well if we go to that, illumos, or rather Solaris had zones even earlier in 2004 or so.
I run illumos and *BSD (FreeBSD servers, OpenBSD on SPARC and network appliances) in production mostly because they just work(tm). They are battle tested pieces of wonderful software engineering with great documentation and community support (ymmv) out of the box. SmartOS has replaced most of our linux and vmware based hypervisors at $work and they are just so much nicer to administer in the simplicity and design compared to the others. In addition we have a Triton cluster as our primary on-prem cloud service.
tl;dr They have proven to be a lot nicer to run as well as more dependable than the alternatives.
By the way, what is your primary motivation for using illumos? genuinely curious.