The reality of a beauty product review is there are a lot of limitations, even for the most honest reviewer. One inherent problem is that many products work differently on different people. Always keep this in the back of your head when watching or reading a product review.
One makeup product that has a highly subjective quality is mascara. This is because mascaras perform differently on different types of eyelashes. People have all kinds: straight and sparse, thick and curly, long. The objective flaws in any mascara: easily clumps, dries out quickly, bleeds to the lids or under eye area. Comments about it being volumizing and lengthening need to be considered in the context of the reviewer and how her natural lashes compare to yours.
Fragrance is also a tricky product to review. This is because it smells differently on different people due to body chemistry. Objective qualities to review for fragrance are its lasting power, how strong or weak it smells when applied, and its notes (floral, fruity, etc). If a reviewer is simply saying the perfume smells wonderful, that's a subjective call. Test the product on yourself and wear it for a few hours to see how it works with your body chemistry.
Hair conditioner is similar to mascara: As someone with naturally thick and coarse hair, I could never accurately review a conditioner made for fine hair. It wouldn't perform on me the way it should. Look at whether the reviewer has a similar hair type, or if it's a written review, make sure he or she explains hair type and if the product is designed for your hair. Also, don't be seduced by products touting vitamins or pro-vitamin complexes. Hair is dead cells, so it cannot metabolize any vitamin. Vitamin E, for its moisturizing qualities, is the only vitamin that can externally impact hair.
Anti-aging products: Even well designed anti-aging products impact people differently and improve skin at different rates. I don't yet have wrinkles, so I can't evaluate an anti-wrinkle cream's performance. There is simply nothing measurable for me to evaluate. When I see the slew of very young beauty gurus on Youtube touting the wonders of Lancome's Genefique, I can't imagine what performance yardstick they're using (other than a paycheck). Objective qualities in an anti-aging product include ingredients: look for established anti-aging ingredients that have scientific backing, such as retinol, stabilized anti-oxidants, and alpha hydroxy.