Can the Halfbike go down steep hills?
I get a high frequency wobble around 17 mph but it doesn't affect stability much. I have, however, had some big unexpected jerks while riding at speed. They're unsettling, but as long as you stay firm in your feet and hands the Halfbike has a way of coming back underneath you.
Descents feel fine to me, with the occasional scary wobble. But again, just keep your body firm and the trike will correct. The most dangerous part of descents is a general safety issue with the Halfbike: the brakes are relatively low power and are dependent on the rider keeping his weight low and posterior in order to work. This is something I'd recommend you practice whenever possible; coming up on a stop sign? See how hard you can brake. Practice, practice, practice.