What separates a rub that builds bark and depth from one that just adds salt to the outside of your food.
A premium rub is judged long before it hits the plate. Open the jar and you should smell individual ingredients, not a single flat wall of salt. That separation is the first sign the blend was built rather than bulked.
Grind matters as much as the recipe. A coarse grind survives a long cook and forms bark; a fine grind dissolves into the surface and disappears. Most blends that disappoint on ribs were simply milled for the wrong job.
The second test is balance over time. Sugar caramelizes, pepper sharpens and herbs fade. A rub built for smoke accounts for all three, so hour six tastes intentional instead of burnt.
Finally, look at what is not in the jar. No fillers and no MSG means the flavor has to come from the ingredients themselves — which is harder, more expensive, and exactly the point.