Wrong! No different than romex in a crawl space.
If a crawl space, basement, cellar, or other similar space is exposed or subject to excessive moisture or dampness, then NM-B should not be used as a wiring method. [Refer to 334.12(B)(4)]
I realize full well that this topic is "subjective", yet I always join the ranks on the side that recognizes the heightened risk associated with NM improperly exposed to excessive moisture or dampness.
When a space is subject to freezing and thawing, such as the underbelly of a mobile home in my part of the world, adverse exposure to water from condensation is enough to substantially contribute to corrosion (due to contaminants, minerals and particles in the water) of the equipment grounding conductor within the NM-B cable. The cable need only be exposed at its termination points such as a heating tape rececptacle box under the mobile home, to be subject to corrosion. Exposure to water in the form of moisture can cause corrosion and insulation damage. Devices, such as receptacles can be adversely affected by contaminants and sediments contained in water moisture.
The more sophisicated an understanding of the damaging properties of corroded, rusting and poor non-thorough connections, the more attention paid to safety.
The underside of a mobile home is built outside of the building envelope, which typically means that the space is considered outdoors and the wiring method should be other than NM-B, such as NMC.