If passing to the general public is the concern, the human face is replete with signatures of sexual dimorphism. Their importance, in my opinion, is as follows:
Very strong indicators:
- brow bone prominence
- lower chin projection, chin length, square vs. rounded chin shape (viewed from the front)
- angle and depth of the nose
- vertical ratio of facial thirds; from the hairline to the eyebrows, from the eyes to the lower nose, lower nose to chin
- upper eyelid exposure (covered / not covered by brow bone and its soft tissues)
Strong indicators:
- anterior cheek bone projection
- upper and lower lip ratio, upper lip curl
- hairline shape
- overall nose size
- jaw width
Mild indicators:
- cheek fullness
- angle of jaw line
- lip fullness
- length of the philtrum (the area between the upper lip and bottom of nose)
- lateral cheekbone projection
Very mild indicators:
- canthal tilt (the angle between the fleshy inner and outer bits of the eye)
- cheekbone height
There are indeed women who carry a lot of masculine facial traits and men who, likewise, carry many feminine traits. The reason that humans maintain a strong ability to tell one another apart is because these differences are often measured in millimeters and depend on how they relate to the rest of the face. For example, a woman with a strong brow bone is often either older or has considerable upper eyelid show. She is also likely to inhabit many other female-range features (like a sweeping nose angle and shallow upper nasal depth). A man with a petite, dainty nose likely still has strong nasal depth, modest upper eyelid show, and a facial ratio that favors the middle and lower thirds. He could also have masculine lower chin projection. The list goes on.
Whether or not a trans woman "needs" FFS to pass depends on how all of her facial features relate to one another. Even one well-established masculine feature can throw off the rest, especially if it is a feature that is strongly associated with sexual dimorphism.
As an aside, facial thirds cannot be manipulated by medicine that is practiced by the vast majority of surgeons (and certainly not any FFS surgeons, that I'm aware of). The techniques currently available to move these bits are fairly risky, often produce ample scarring, and are almost entirely reserved for people disfigured by genetic abnormalities. However, someone with masculine thirds (especially a very long midface) can still easily pass as long as their remaining features are feminine, hence there are few people that skilled FFS cannot help.