Unstable structures? Yes on unstable people, but I wouldn't go so far as to call Civil Center, Tenderloin and SOMA no-go zones, especially in the daytime.

Civic Center is, as its name suggests, the cultural and political center of San Francisco. City Hall is an especially beautiful building. I wouldn't put Civic Center on a must-see list for tourists, but I would still recommend it if you have a few days in town. Same goes for Yerba Buena Gardens/SF MOMA area in SOMA.