I'm a graduate student in Environmental Fluid Dynamics and I deal with a lot of environmental sensors. The temperature sensors on phones are pretty accurate, but they're quite sensitive to small changes. This means that nradov's original statement is correct: Body heat and cell phone usage are absolutely going to affect the temperature sensor. For example, Sensirion makes a high quality temperature/relative humidity sensor and they recommend the PCB have cutouts around the sensor to offer proper ventilation and minimize heat transfer from surrounding components [1]. On the iPhone 6s, the pressure/temperature sensor is on an edge, but it's also right next to the NAND flash chip, so I'm not quite sure what the heating effects would be.

I can't answer much to your second question but I imagine it's easier in colder environments, especially if the phone is not in a pocket. In addition, here's a publication that claims to be able to get useful signals from noisy data, so I don't know if I would say the measurements are useless [2].

[1] Fig. 10 in https://www.sensirion.com/fileadmin/user_upload/customers/se...

[2] http://onlinelibrary.wiley.com/doi/10.1002/grl.50786/full