There are certain products that I think a lot can be said from limited testing, but I think that, at a minimum, products have to be applied as intended in order to determine pigmentation, whether it’s buildable, blendable, how it sits on the area, whether it gets patchy or fades initially, is there fallout, and so forth. If someone normally wears eyeshadow primer, I don’t know that longevity tests are game-changing since most eyeshadow performs very well over primer.
However, for other products, like foundation or lip color (as two examples), how it wears/breaks down is critical, whether it’s drying/hydrating, and so on are all things that can’t be known without at least wearing once. I also think that it’s more important to limit just how much new is incorporated, e.g. new primer, new foundation, new concealer, and powder together makes it harder to see what’s doing what and where compared to using one’s go-tos and then a new powder.