On a group trip, a sales person from the local outfitter took one of the store's hammocks for a trial run (I think it was an ENO); we got a light rain for a couple of hours during the night, and he got soaked -
When I first started using my hammock I didn't have a tarp so I just took a plastic tablecloth and set it up more to keep dew off me than rain. I depended heavily on the weather forecast, but I knew the odds were against me and I would get rained on. Here in the Ozarks that often means a fast moving front with strong winds and hard driving rains. The storm might only last 10-30 minutes, and most often the sun or stars will be shining bright a few minutes after it passes over.
So, before buying a tarp I looked into several designs, knew that some of them, despite the claims of keeping you dry, simple would not, here, in the conditions we face. This 12'x12' design did though. I tested it in two, back to back, pretty vicious storms. Even the leaf mulch under it was dry. It stayed pinned down, nothing came loose, and there's room for a pack under it. I really don't see how anything much less than this can be expected to keep you dry.