Its almost all open software, that helps a lot. They add a minimal amount firmware, rather then the many, many million lines of firmware that is usually around. And most of the stuff they added is Rust on a micro-kernel. (Check out the talk I linked top level to see some of their low-level debugging infrastructure).

To bad they can't (yet) get open firmware in the NIC, the SSDs and some of those other places (Time for an Oxide like company that makes P4 driven NICs). But nobody else can really offer that either.

The only real issue for them is that Illumos is the host OS. Its open source and stable of course, and has good debugging tools. But in terms of industry experience, the amount of people with deep knowlage of the system are harder to find compared to Linux.

The of course also add some complex software on top that will have to work properly, moving VMs, distributed storage and so on.

Full DIY is pretty damn hard, you need a serious team to pull that off. The Dell VxRail/VMWare is the more reasonable competition. I think VMWare going full Broadcom mode will make them more interesting. Buying into that ecosystem isn't that appealing right now.