Flat is even worse.

Consider this: In addition to headwinds, tailwinds, crosswinds, etc., Strava doesn't know if you're sitting bolt upright or in a time trial position. Doesn't know if you're wearing a parachute or a skinsuit. Doesn't know if you're riding on pristine pavement or rough chip and seal. Doesn't know if you're riding Vittoria Corsa Speeds or Gatorskins. All of which significantly impact your speed and the power required to go at that speed.

Sure, you can mitigate all of those with identical setups each time and the like, but at that point you're better off just using a calculator so you can specifically account for all of those factors. Or just use time itself. Probably easiest!