In each city it depends all on the neighborhood.
San telmo is SOOOO dodgy. The last time I was there at night (a Thursday around 10:30), I walked with 2 girls. There was a drunk guy shouting nasty piropos at us from his corner cafe outdoor table. Halfway down the block we turned around and he had followed us down the street to say more of god knows what. I shouted at him to stay the hell away from us. A few seconds later there were some dodgy characters looking like they were about to jump us. Then a crazy old lady blocking our way in a tight spot on the sidewalk with her hand held out shouting "moneda! moneda!" - we re-routed around a giant garbage bin to get passed her. Then just a block before 9 de Julio, someone crouched down in a stoop of a doorway smoking something out of what looked like a crack pipe.
It was like a tricky level of Grand Theft Auto or some other f'ed up video game. All in the course of 5 minutes! Never going back there at night again (unless I take the car and can park within half a block of wherever I'm going). I'd be terrified to go alone too.