To be fair, they are built to a price point. The bags they are using have approximately 5" of travel between fully extended and bottomed. A typical pickup has 10 or 11" of travel in the leaf springs. You do not use 11" of travel for bumps (typically), the springs are made that soft so that the ride isn't unreasonably harsh when empty, yet can still carry a load by compressing them. This compression is the sag that air bag helpers attempt to limit. The problem is the bags must be set at a height that does not overextend them when empty, nor bottom them when loaded, and this is a large compromise: if set to not over extend, about 1/2 of the load carrying ability of the steel springs cannot be used as the air bag bottoms. If set to use all of the load carrying of the springs without bottoming, they will overextend when unloaded. You are instructed to install them in the former way, but then must run higher pressures and higher loads on the air bags to keep from bottoming them. That results in the high and very progressive total spring rate that people complain about with air bags.
A related problem is that the stiffness of an airbag is proportional to its installed height for a given load carrying ability. On true air suspensions the bags are 10" or more high. It isn't possible to do that on a pickup using the inexpensive brackets that these kits use.
I've installed 6 of these kits over the years, they have their place in eliminating sag for not much money, but they come with significant limitations. On my current truck I installed true air suspension and the difference is huge.
