I think some people set speed run records on the steam deck. Obviously, the number of pixels is lower and the processor isn't as powerful as a desktop, so a megafactory might not work well, but should be fine for tinkering with a design.
I played a whole run of the base game without mods on my Steam Deck in handheld mode and found it just fine. It uses the extra buttons on the back quite effectively.
Imo it's a little too cumbersome to comfortably play on the deck. Totally possible and it does work thanks to community controller presets but I feel limited playing on the deck compared to when I play on mouse and keyboard and a full size monitor.
The Factorio dev. I've heard he's a shit person but not dove deeper into it.
Edit; did some looking and yeah, he's a shitbag. He claimed that Statutory Rape was a SJW term, and that teachers should be able to seduce their students as long as they weren't violent.
Then he used a dev blog to link to a racist misogynistic jackass, and when called out on it (The guy just wanted a disclaimer on the link) Korvex went on a rant about cancel culture and then tried to defend the misogynistic views, claiming that "maybe there's a reason why there are so few women coders"