I suspect it would be possible to go lighter with a bivy. With tents vs. hammocks, I've seen comparable ultra-light options. And of course there's the need for somewhere to hang.

But for me the only real issue is comfort. I have always had a hard time getting comfortable in any ground set-up. Hammocks are the opposite. I'd say that if I hadn't found out about hammock camping, I would never have gotten back into bike touring.