OS Comparison

Summary

Arch Linux based.
Defaults to Big Picture Mode with the ability to switch to Gamepad/Deck UI via a terminal command.
Uses Gnome for desktop mode.
Uses an A/B atomic update system for stable updates.
Uses a read-only file system with an option to temporarily switch to read-write mode via a terminal command.
Does not allow for custom partitioning.
Works on almost any PC and has specific support for many handhelds.
Monthly releases that always use the latest vanilla Arch Linux packages.
Includes the ability to remotely install software without using desktop mode and manually adding Steam shortcuts.
Includes out-of-the-box support for Epic Games Store and GOG.
Includes many pre-installed emulators.
Includes automatically applied game tweaks for additional game compatibility.
Includes functionality to automatically download missing artwork for Steam games in Gamepad UI mode.
Includes GE Proton out of the box.
All code is available in the GitHub repositories.
All infrastructure is automated and hosted on GitHub making it trivial to fork, build and install a custom version of ChimeraOS.

Arch Linux based.
Defaults to Gamepad/Deck UI with no option for Big Picture Mode.
Uses KDE Plasma for desktop mode.
Uses an A/B atomic update system for stable updates (differs in the underlying implementation).
Uses a read-only file system with an option to temporarily switch to read-write mode via a terminal command.
Does not allow for custom partitioning.
Currently only works on the Steam Deck.
Often includes stale software with many patches to base Arch Linux packages.
Additional software must be installed from desktop mode and added to Steam manually.
No out-of-the-box support for Epic Games Store and GOG.
No pre-installed emulators.
Missing Chimera provided game tweaks.
Has missing artwork for many Steam games in Gamepad UI mode.
No GE Proton included.
Source code for many components does not appear to be hosted on any public repositories.
May require additional infrastructure to fork or customize, including your own build system and hosted Arch Linux package repositories.

SteamOS 3.x/Arch Linux based.
Defaults to Gamepad/Deck UI.
Uses KDE Plasma for desktop mode.
Does not use an A/B atomic update system. This could potentially lead to a broken system in the future if there is an update that cannot be applied automatically. This problem is exacerbated when using any packages from the Arch Linux repos.
Does not use a read-only file system, allowing for the potential use of the Arch Linux repos. However, doing so comes with a high risk of breaking your system.
Allows custom partitioning.
Works on almost any PC with an AMD GPU.
Often includes stale software with many patches to base Arch Linux packages.
Additional software must be installed from desktop mode and added to Steam manually.
No out-of-the-box support for Epic Games Store and GOG.
No pre-installed emulators.
Missing Chimera provided game tweaks.
Has missing artwork for many Steam games in Gamepad UI mode.
No GE Proton included.
Source code for many components does not appear to be hosted on any public repositories.
May require additional infrastructure to fork or customize, including hosted Arch Linux package repositories.