@mysterio

From the point of view of what the wearer has to do, I believe that to obtain a good hairline you have remove as much lace as possible,
but not cut it back so far that you lose the random scattered hairs along the front.
You also have to be accurate about matching up the line of adhesive with the front of the unit.
People have great success with tape, but I find a couple of layers of glue really helps lace sink into the glue & become invisible.

From the point of view of what the unit should be like, a graduated & well ventilated hairline really helps.
It has to have enough hairs to disguise the base material, but still have a lower density that shows some scalp.
The knotting has to be good and well bleached, especially for darker hair colours.

It should also be remembered that indoor pictures, taken with a flash, tend to blank-out a lot of what you could possibly detect under normal lighting or even daylight.
The over exposure can sometimes hide a multitude of sins and therefore not necessarily a true representation of what it actually looks like.