I have come to belive that true-underexposures are almost always caused by "us". It is caused by outright mistakes in setting the camera, poor metering technique, a lack of understanding of what the meter is saying, and or in trying to cheat to avoid camera support or flash when it is clearly needed.

"We" are the wild card, not the film. I think it is actually almost impossible to get an underexposure caused by the film ISO rating. Ilford, Kodak, and Fuji are very good at their jobs, their films are well made and consistent and as best I can tell work extremely well and deliver exactly what they advertise.