Bazzite and HoloISO are immutable, that's a huge difference (good or bad, depending on your use case), and If you don't care about VRR & HDR, Bazzite also supports GNOME the other two distros don't.
Also, out of the three, as far as I know, Bazzite is the only one that supports Nvidia.
I also need to evaluate which distros have the native Steam packageor if they use the Steam flatpak, since the Flatpak version doesn't support libraries on multiple drives. That would also be a huge difference.
That's the main difference between Holo and Chimera. Their upstream setups with SteamOS are essentially the same.
Also, Nvidia drivers don't work with any of the extra Valve tools in SteamOS (gamescope), so if you're going the Nvidia route, I wouldn't recommend using anything based on SteamOS anyway.