I am particularly impressed by the easiness of the update process.
FreedomBox and Yunohost use more traditional software installation mechanisms; they'll install packages, run scripts, etc. They just add (sometimes very nice) UI around it. While that's great for some things, after a while things can get a bit messy. For example: what about when a package installation fails for some reason? Or one of the configuration scripts fails? Well, you're stuck logging in and troubleshooting, which isn't super fun (and might be intractable for less technical users).
Sandstorm, though? Everything is sandboxed and isolated from the rest of the system. Everything. Backing up or restoring an instance of an app is a few clicks in a web interface. Sandstorm handles auth so the app doesn't have to... etc etc.
This has its downsides, namely that apps that aren't written with this sort of usage in mind might not fit in as well. But for those that are, it's by far the best experience I've had. I have Yunohost and FreedomBox servers in varying states of disrepair, but my Sandstorm server keeps chugging along. Big fan.
FWIW, there are places Sandstorm could improve here. Probably the biggest one for me is that Sandstorm backups do not happen automatically in the managed space. (You could automatically back up your Sandstorm server with another utility, and you can manually backup/restore individual grains in the web UI, but there isn't yet a really clean integrated way to restore grains inside Sandstorm.) But if this is the one thing you have to figure out outside of Sandstorm itself, that's not too bad (or unusual for many server applications).
Also, the parent suggests being able to offer a hardware box good-to-go, and I'd like Sandstorm to have that, or at least, a full distro release, where you do not have to worry about the server OS at all. It's something we've talked about quite a bit.
That would be me. I've done it on a private app and helped bring it to another app, so it's repeatable. I'll try to explain it on the sandstorm-dev group in the next week or so.
Something along the lines of someone buys some hardware with this platform on it and gets a gui that lets me install "apps" on top of it.
Personally, I've got a home setup that is on its way to what the op has; but I think there's demand from non-techy folks to get off the big co's apps and onto privacy focused ones that they control.