Suspended over an active volcano perhaps, can't think of anywhere else where I couldn't use a tent.
Swamp? Filthy urban "woods"?
I've slept in all kinds of shelters. The hammock is nice because you can stop caring about flat ground, wet ground, etc. Ground based shelters will work even if the tallest tree is a 2,5 m birch. A tarp has the best space/(weight x work) ratio, but is annoying in mosquito season and often lacks in privacy, e.g. if you have company of the appropriate sex, or are out with people who care about (or stare at) skin. A tent is a confined little cubbyhole, where you can't see the world around you, but it is a very sheltered and private little cubbyhole, even in extreme conditions. Snowshelters are ever so warm and protected, but they take a lot of time and effort to build. A debris hut (or other natural shelter) takes time, and you need permission to gather materials, but can be quite nice for the effort. A big log fire is warm all night, but is even more tricky in some landscapes than a debris hut. In an hotel room you may have a Bear for neighbour.
All shelters have their advantages and disadvantages, all have places where they are one of the better choices, all have places where they are an insane choice.