I do agree that excessive flux depth or too shallow of depth could cause porosity, but these conditions are quickly adjusted when setting up the equipment.
Excessive depth is usually very easily controlled so I don't think of it as a problem; one can quickly see the roughness and unevenness of the weld and adjust the flux depth. The same for too shallow of a flux burden one see the arcing and adjusts the flux depth.
The problems I have seen have been related to a poor weld profile trapping the fines along the toes and then adding a pass on top.
Definition - Recycled flux is the slag that is collected from the weld send out to a recycle, ground up, adding silicates and mixing with a percentage of virgin flux. also known as crushed slag.
Using recycled flux that has not be properly screened (eliminating the large and small particles) is a problem, many recyclers can screen out the large particles but the system they use for removing the fines (generally an electrostatic system) only removes about 1/3 of the fines, either the fines are have a reverse charge than the system or they have no charge at all (Gaussian distribution). I must admit some ground slags are worst then others in generating fines. The size of the silicates added to the ground slag needs to be monitored.
If the filtration section of a flux recovery system is not working correctly, you might obtain excessive fines, however, this should not be a problem if you are doing PM on your equipment, and know how to operate your machine.