There are a number of techniques for injecting fillers: linear threading, serial puncture, depot, and fanning. They are used separately or in combination depending on the area to be filled, the filler used and the goal one is trying to accomplish.
The fern technique was developed to get a smoother result in dynamic folds, like nasolabial folds, so that the filler is not more apparent when the patient is smiling or talking.
I use a combination of techniques to achieve the desired cosmetic goals.
For the lips, linear method along the border of the vermilion to outline the lips, then adding small boluses in the four lip pillows, and linear along the philtrum work well. Injecting filler in a K shape at the corners of the mouth buttresses the corners if they are downturned or deepening, brings the corners up.
For restoring the volume in the soft tissues around the lips, linear threading or fanning somewhat parallel to the lip border with small amounts of filler gives lift to the surronding tissue and to the lips themselves. Filler must be done conservatively around the lips while preserving or restoring the proper proportions of upper lip to lower and the distance of nose-to-upper-lip vs lower lip-to-chin. It's not just a matter of which injection technique to use.
The fern technique was primarily developed for the smile lines, so that the lines don't show up above or below injected filler. This is a good technique, but again multiple techniques can be used to accomplish this goal.