Niacinamide is a genuinely good ingredient, and this made it available to everyone for six dollars. That matters.
The evidence for niacinamide is real and it is specific: Barrier function — it increases ceramide production and reduces water loss. Strong evidence. Hyperpigmentation — it interrupts the transfer of pigment into skin cells. Strong evidence, from a 12-week placebo-controlled split-face trial. Oil control — measurable reduction in sebum. Good evidence. Redness — moderate evidence, largely downstream of the barrier effect.
Zinc PCA adds a second oil-control mechanism, which is why this suits oily skin specifically.