Mexico City

It depends on where you sleep and how you get around. In Roma, Condesa, the Centro and Coyoacán, hostels run something most nights and meeting people is easy. The real risks are unlicensed taxis, express kidnappings and spiked drinks, all spelled out by the FCDO. It works well for people who stick to app cabs at night, far less for those who improvise.

Buenos Aires

Yes, if you are happy to eat late and keep your phone in your pocket. Daily free walking tours, milongas with a class before the dancing and hostels that run asados and football trips make it hard to stay alone. The real risk is snatch theft, including from motorbikes, in areas the FCDO lists by name. Peso prices change monthly, so recheck them before you go.