One thing that Steam/Valve has done with the Steam Deck is lock down the ISO by default, and provide no tools to modify your image persistently. That is of course on purpose, because that works for 99% of users, but the 1% of users may wanna use something where they can, for instance, overlay packages and keep them with updates, or apply extra gaming-focused tweaks that may be more of a hassle to maintain on SteamOS.

For instance, I use Fedora Silverblue daily on my Desktop, and even though it is immutable just like the Deck, it offers me tools to modify my image as I see fit and have the same modifications be applied to future updates too.