In my experience, product. There’s a pervasive view that product is easy, that anyone working in it is a dunderhead who gets in the way of engineers doing the really worthwhile work. In reality, it would just mean they rewrite the software five times and have zero customers.
Oh, design too. I’ve met many engineers who simply don’t see any value in design.
This. I'm a former teacher. I left to get a PhD in educational psychology, and now I'm a data scientist for a large school division. The number of ed tech companies I interact with who clearly have little grasp of research-based best practices in education is staggering. But they can all build slick-looking websites and dashboards.