No, the north east of Yucatan is not the only relative safe place for gringos. There are more scattered all over the country. The most popular are Puerto Vallarta, Puerto Escondido and actually the whole of the state of Oaxxaca, Campeche, Queretaro, Monterrey, parts of Mexico City, Guadalajara, Cabo san Lucas and specially the state of Yucatan where Merida is located and known as the safest city in Mexico. I would even go as far saying that anywhere except the northern border areas, Sinaloa, Durango, Chihuahua, Guerrero, Colima and Jalisco I feel safe and no problem to live as a gringo.