Software engineers drastically underestimates GUI - Web layouts, mobile app layouts, and even PDF layouts are non-trivial pieces of work to get right in all circumstances.

Nobody who has actually worked on those things think that. You might want to qualify if you're only talking about people who have never worked in this area.

In my experience it's the NON software engineers who tend to underestimate the complexity

Yep, they (can) rarely enter your domain... so it's easy to assume its going to be trivial (maybe because things like .md or .txt files are trivial, so it's easy to think there's not much of a delta)