Several caveats to go with urban homesteading, most all having to do with code enforcement. Then there's the issue of water collection again...

In most cases, it would be nothing more than novel due to the limits imposed. Imagine trying to get away with planting your own garden in, say, Central Park??? There are community garden areas around, but my experience is they are either too crowded or too expensive or too hard to access to be worth the trouble.

Suburbia still seems to be the best compromise. If you don't have a yard to work with, don't expect much.