I guess I lucked out during a 9-day stay in Delhi in January without any ailments. I stayed with relatives and mostly ate home-cooked food. When we ate out, it was places that my relatives were familiar with, such as the Bengali Sweets House in Bengali Market, Kamal's in Sundar Nagar, a smaller non-veg place in New Friend's Colony, etc.
I did get a lot of sweets and chai from street vendors and small snack shops in Bhogal, Janpath, New Friend's Colony, Chandni Chowk, etc. and they were all delicious -- and best of all, without any "side-effects."