Regenerative braking is nice but as sibling pointed out it doesn't nearly recapture everything. Another factor is that the additional load whether accelerating up a hill or braking down one puts more current through the system. In fact nearly every part of the drivetrain will experience accelerated wear under those conditions, mechanical or electrical. Cooling systems, too. Properly spec'd, it's of course very doable, but it's true hilly terrain is more difficult.