They are not scary at all. They feel very secure, on the contrary.
I have only once had an "over" incident with one, and the horse pretty much flipped over backward on me (young stallion pitching a fit over being left behind).
I curled up my knees and neatly laid him aside, after which we got up (he was still securely harnessed and in the shafts) and trotted off. Taught HIM not to pull that trick again.
I guess that to me illustrated the worst case scenario, and it really wasn't frightening at all. The strength is such that nothing broke even under that unusual load (we've had tires up on curbs, tires driving up the sides of hills/walls/fences and never come close to turning over), and the horse was just fine, so I figured the best thing to do was move forward!
I wish you were closer, as I am all about sharing the Hyperbike with anyone that wants to try before they buy. I have never found anyone that didn't like it, though they do have physical limitations, I even have a friend that has had one modified for her disability (she uses a wheelchair, normally, and her Hyperbike seat swivels).
As to discomfort, I don't find it at all uncomfortable. I used mine for hours on end while I was 8 months pregnant and had no back pain, no leg pain, etc. I guess THAT was my worst case scenario on THAT aspect. *LOL*
Here's a photo of my Hyperbike, hitched to my little gelding's grandpa, Trigger (Cherry Bomb's daddy for those of you that have met or know her):
I was pregnant with my second son in this picture. My cart is also featured on the current website near the bottom of the pictures showing the wheels being taken off and on. I have other pics around here, but will have to look them up. Mine is the older style with the smaller seat and I love it a lot, but the new ones are really comfortable, too!
Liz