What is the downside to installing faced fiberglass insulation in a dirt floor crawlspace vs non-faced?
"When fiberglass batts are installed in the ceiling of a crawlspace, gravity always wins in the end" source

Faced or unfaced, it is almost always a bad idea to insulate the floor above a crawlspace, especially with fiberglass batts. Insulate the crawlspace walls and seal the vents instead. The one exception is if you are in a flood zone.

A sealed crawlspace will be drier, healthier, and usually more energy efficient than one insulated at the floor.