It was always running on a free base (Slackware). Unraid sells because of its ease of use and reliability. It's selling the features on top of the OS. It wouldn't be as popular or as functional as it is if it wasn't running Linux under the hood.

If there is one piece of software I've not regretted buying a lifetime license for, it's been Unraid.

It may not be for everyone. And it has its quirks like anything else. But I've been running it for years now and have even had to experience the dead drive recovery process. It's solid.

Could I use something free? Probably. But, I'm getting old. I just want something that has good community support, has an nice interface, and also still allows me to get my hands dirty with bash or programming when I need to.