It does look like a single brake and on the front wheel at that. That could be a serious objection. The other brands like Footbike and Kickbike do have two brakes, and they look like they have a bit more road clearance.
Downhill, though? No. Hills go up, not down. Just like winds are always from the front. Seriously, you spend much more time going up than down, so I doubt you would miss it.