The procedure works because of donor dominance: follicles at the back and sides of the scalp do not respond to dihydrotestosterone, the hormone behind pattern hair loss, and they keep that property after they are moved. That is also its limit. The donor area is finite, it does not regenerate, and it is spent once. Which is why a graft count should be a calculation after examination rather than a number offered before it.
It suits a stable, patterned loss with an intact donor area, someone rebuilding a hairline or a crown, and, with DHI, densification between hair that is still there. It suits less well a diffuse thinning where the donor area is itself affected, a loss that is still moving quickly and has not been stabilised medically, and an expectation of the density of a twenty-year-old head. In women the pattern is usually diffuse, and iron, thyroid and post-partum causes belong ruled out first: otherwise surgery is performed where hair would have returned on its own.
We turn cases down, and that is a feature rather than an apology. A plan that says yes to everyone is a sales process wearing the clothes of a medical one. If your donor capacity does not support the area you want covered, the honest answer is a smaller plan or none at all. Said before you book a flight, not after you land.