Eh. That goes for everything.
If an engineer designs something, it tends to be complex, regardless of whether or not the designer has “the big picture,” because they design the whole thing to fit their worldview. Engineers have a complex worldview.
If a UI designer designs it, the chances of it being simple (from a user PoV), is much greater.
Designing a simple UI, in my experience, is often “un-simple,” however. I remember writing a visual cropping algorithm, for a film scanner.
It was a pretty damn hairy library (it ended up being an entire subsystem).
Worked well, though, once I refined and debugged it.
The UI designer might be capable of designing a simple UI but probably has no idea how to design a simple infrastructure, dataflow, logistics, corporate operations infrastructure, finance structure...
You're thinking like a UI designer that's watched engineers without UI design experience try to design a UI. You're referencing a lack of expertise and experience, not some inherent trait in engineering.