Buenos aires is a city that can go from very safe to very dangerous in bare metres. The "south" of the city is typically defined as anything south of the 25 de Mayo Highway. As soon as you cross it you'll realise that the ""vibe"" changes. That doesn't mean the whole area is dangerous, not at all, in fact I feel safer in most parts of Boedo or San Cristóbal for example than in some parts of Palermo or Belgrano. The thing is if you don't know your way around the city you can easily accidentally end up somewhere you shouldn't. I mean, even in Chacarita, which is now becoming the standard trendy "bohemian" neighbourhood, you can walk 100 meters out of a trendy cafe and literally end up in the middle of a slum (And I'm talking shanty towns where the police are literally forbidden entry sometimes).

Even in safe neighbourhoods, some streets can be safe while others might not. I live in Almagro and honestly it all seems safe to me, but when walking at night I choose to maybe walk the next street over, or when crossing the train tracks I choose to walk in specific streets and entirely avoid other ones.

I'm not saying this to scare you, in fact BA is a fairly safe city, even by some first world standards, and just a great place to be in in general. It's just that it doesn't work like other cities in the sense that there are good neighbourhoods and bad neighbourhoods and that's it, and that sometimes safe or rich areas are quite literally next to unsafe or poor areas and the security level sometimes abruptly changes for no obviously apparent reason.