I meant to reply to this but didn't, sorry.
Take Indiana Jones and the Great Circle - a game which can be a challenge to even high-end PCs.
Its got its own special Steam Deck settings which can be summarised as "lower than are possible to set on PC". They look OK(ish) on the Steam Deck at 720p and for the main part it delivers 30fps+. When docked it looks appalling on any display which is higher-res as there's not enough resources to upscale. Doesn't matter how far away you sit it isn't getting better. I've been short-sighted since I was a child and it still looks crap effort
The Steam Machine will run it at 1080p internal resolution with medium settings and upscale to 4k at 60fps. Looks fine when you're sitting 2 metres away.
I have no doubt it could look better with a PC. I have no desire to maintain a PC (whether Windows or Linux) in the living room and SteamOS has been as hassle-free on the deck.