I gotta say IMHO the HCH is fine on hills, taking advantage of the assist up and regen-down, but you'll always win the FE race on a flat course. My FE is always a bit worse when I tackle even moderately hilly terrains, in round-trip situations, of course. Not that I have many flat areas around here. Just relatively less hilly

When I think about it there's some less obvious advantages to the flat terrain, such as visibility and less frictional losses due to generally straighter roads (vs windy hilly areas). ANy other hidden ones I'm missing?