eh, from my experience the only plant that has "massive" difference between strenghts is the one you buy in backalley I tried about every plant with mood altering qualities from esoteric/health stores and I picked many on the meadows.... and it's not "massive" difference.

(when it comes to st john's wort it's apparently important HOW you pick it... but small sellers probably know the best way).

And it's better to use dried/powdered plants than the pilled ones. Just make a damn tea and you'll be fine. Yes, valerian tea (I macerate it in cold water for few hours for strenght) tastes gross. But as you said if person has trouble they will deal with many things to overcome it.

Also... there are many producers of plants/aromatherapy/homeopathy... so on, that strongly believe in fair trade principles and that it has effect on the final strenght of the product. You can look them up if that is your problem. You can't really put Pharma and fairtrade in one sentense...