I worked as an engineer in…
I worked as an engineer in this industry for several years, both on the aftermarket lifts side and on the OEM parts side. OEM suspensions are developed on the scale of years. Most aftermarket lifts are a race to be first market, and development is done in a matter of weeks. Think about that for a minute. Most aftermarket lifts are not designed by race-experienced offroad wizards. They're developed by company engineers who take measurements of the truck and then design parts that fit to achieve the intended lift. They try to not compromise the geometry of the truck too much, also working around various ABS and stability control systems, but also can't totally re-create and validate the whole thing; nobody would spend the money for a whole new suspension.
1) even low-budget aftermarket lifts are in most cases safe for street driving. They are not typically designed to handle serious offroad use any better than the OEM suspension.
2) lower-budget aftermarket lifts try to maintain stock suspension geometry, but often times some compromises may be necessary. It typically makes sense to install the whole kit; including the available optional parts (even if it means ordering a couple 'companion' kits, like in this article) if you plan to lift your truck.
3) even with all the available parts installed, get the truck aligned, seriously consider having an offroad shop inspect (or better, install) your kit for you.
4) trucks that are already factory lifted are often still using the base truck's control arms and therefore are using much of the available travel of parts like ball joints and links. Adding additional lift on top of this may push these parts beyond their design envelope.
5) adding a lift will typically make the truck less dynamically stable (higher CG) and less fuel efficient. Assume it will ride differently, and most often not in a good way. Parts may wear out more quickly, as the increased height can increase loads on various suspension and driveline components.