It's not the pigs themselves - it the huge holding pits, like huge swimming pools, full of their wet manure. Pretty nasty... Pigs themselves, if given correct housing, are clean animals...
I realize that, having lived on farms in NW Illinois during my first 62 years, and raising hogs the old way (portable sheds and farrowing barns) in lots, and pasture, for a third of those years.
The outdoor lagoons (except when agitating it for removal purposes) never seemed as bad as the huge holding tank (basement) below the slatted confinement floor. Not sure why. Maybe because the some of the moisture would seep into the soil or dry from the sun and wind, and the top of the lagoon would dry enough to create a seal of sorts.