SteamOS is Arch-based. Arch as a distros is extremely bare-bones. The main difference between all base-distros is how they manage their packages.

Sub-distros may opt to change how the package manager works; Manjaro delays updates until everything has been verified to be working and not likely to break anything. Yet, it is still Arch based.

There's nothing about a base-distro that makes it inherently unstable. Arch is extremely reliable, depending on what you need it for.

Probably because rolling distros get updates for new hardware sooner. The Steam Deck was using a brand new AMD APU at the time so it probably benefited from being on the latest kernel and mesa.

Arch really isn't that bad. I haven't had too many breakages, and when it does break, they're usually fixed super fast or they're easy enough to workaround, whereas I often have to wait weeks or months for bug fixes in more stable distros. Now, would I hand it to my grandma? Not unless I am the one doing the updates for her, but it isn't the ticking time-bomb that non-Arch users like to say it is.

I read this article a couple of years ago about the subject. Apparently, one part was ease and rapidity of rolling updates with the likes of Arch, and another part of wanting a bit more experimental nature of Arch versus steady-as-she-goes and stable Debian. It's a rather neat discussion and debate.

Valve devs said they like Arch because it allows them to update steam deck faster than other distros. Looks like they're skilled enough to limit the downsides of an unstable distro in order to get the latest and greatest package versions.

The Valve devs have complete control over every update that goes out. If they do something that breaks the system then their testing was inadequate. But even when that does happen, they can quickly push out a patch. Also, having just one hardware target to test for makes things a lot easier.

SteamOS is based on Arch but doesn't follow the same release cadence, as well as using an immutable base OS and an A/B partition layout (much like Android) allowing for rollbacks. So the problems that an Arch user may run into are less likely with SteamOS, because it has some extra protections and testing that is done before it arrives on the user's device (particularly with the stable channel).

And gaming distributions often prefer a rolling release setup, because gaming benefits from very recent kernel and GPU related packages (that aren't always fully compatible with older/stable distributions), particularly with AMD hardware.

For the same reason Chromebooks use Gentoo Linux. It fit their workflow in development an immutable OS.

There are a few hang ups an end user might encounter but with an immutable OS it's been thoroughly tested before reaching the user.

At the end of the day Linux is Linux and it doesn't really matter what you build your system on. If properly tested, it'll be rock solid.

Many Valve devs use Arch on their personal PCs.