What's more important is that the team actually knows Illumos/Solaris inside out. You can work wonders with a less than ideal system. That said, Illumos is of high quality in my opinion.
Seems risky considering how small of a developer pool actively works on illumos/Solaris. The code is most definitely well engineered and correct, but there are huge teams all around the world deploying on huge pools of Linux compute that have contributed back to Linux.
They had a bug in the database they are using that was due to a Go system library not behaving correctly specifically on illumos. They've got enough engineering power to deal with such a thing but damn..
Linux grew up in the bedrooms of teenagers. It was risky in the era of 486 and Pentiums. The environment and business criticality of a $1-2M rack-size computer is quite different.
