Your warnings about low-pressure loads are quite on point. There's a certain break-point (for lack of a better name) below which all one will get is a blooper, gobs of plastic spaghetti in the bores, half-burned powder, lots of acrid smoke and a wad hanging out the choke. If you do it right, like I did when I started reloading, you'll even get to watch the shot charge as it flies out the barrel like a rainbow and hits the roof of the trap house, doing no damage to the shingles. And then you get to go, red-faced, to the scorer's chair to get the ramrod to remove the wad from your barrel.
Or worse, you don't notice the obstruction.
That break point is at some point where the pressure gets too low for the ambient temperature and the powder does not burn properly. Smokeless requires confinement to burn properly, and confinement means pressure. So, loading too light can be just as bad if not worse than loading too hot.
