[mgaunard]

Most problems are not embarrassingly parallel.

(Fun fact: I once had someone call HR on me because they didn't know embarrassingly parallel was a technical term, and they thought I was belittling them)

[rfoo]

Prefix scan is not embarrassingly parallel. Yet OP's statement still works when you change it to scanl (+) 0 [0 1 2 3]

[mgaunard]

That requires + to be associative. And scan is one of the core skeletons of parallel skeletons, so obviously if you express everything as parallel skeletons, parallel programming remains manageable.