I had to watch several videos on youtube to answer this question for myself. The advantages to using ChimeraOS over big picture mode seem to be tied to Valve's gamescope compositor, which gives you access to a nice UI for limiting framerates, applying mangohud, and choosing upscaling settings. ChimeraOS also has compatibility with Steam Deck plugins.

I know nothing about the plugins. You can definitely do all the other things outside of gamescope's UI, but... it actually is a pretty nice compact UI.