D2’s BFF volunteered in several medical clinics in Oaxaca over 4 years. And went back there last summer to visit old friends. She never feared for her safety and felt comfortable walking around in the local towns and riding public transit – even cross country buses. (BTW, the BFF is very, very fair, almost albino fair–so she stands out in a crowd.)
The sponsoring organizations are quite aware of the local situation and will not send volunteers into areas they feel are unsafe.