I use a Spongy Wonder seat. The Spongy Wonder is designed to only contact the sits bones. You are right about to be concerned about balance. When switching from a nose to a nose-less seat the transition is jarring. It takes a little while to compensate for the new saddle and shift in center of gravity. I was back to full confidence in riding the bike within an hour.

For someone learning to ride I imagine it wouldn't be any harder to learn than a saddle with a nose.

The other thing about the Spongy Wonder is that it helps with getting back on a bike if you have any pains or issues in the mid-line since there is no pressure there like a traditional seat.

I have a friend who uses a saddle like that on his bike because he has to sit in a very upright position due to his physiology. It works for him and it's very comfortable but I doubt it would work well in a typical riding position on long journeys.

That looks like some seat someone is trying to make up a purpose for. No serious cyclist is putting that on their bike. First time you need to slam your brakes on, you are going to nad yourself on the top tube.

I dunno about the idea generally, but this product specifically is clearly shoddy sucker bait.

This looks like a seat for a recumbent trike that has an independent backrest.