Three kinds of leaf
Some leaves are farmed to be brewed — rooibos, yerba mate, guayusa, sideritis. Some are kitchen aromatics that are also infused, where the drinking use follows the cooking one: sage, bay, oregano, curry leaf. And some come off trees planted for something else entirely: olive leaf from pruning, mulberry leaf from silk plantations, persimmon and loquat leaf from orchards. Which kind a leaf is tells you how it is graded and how consistent it will be.
Drying temperature is the whole quality question
Aromatic leaves hold their character in surface glands, and heat drives it off. Shade-dried leaf that still crumbles rather than powders, and that still smells of itself in the packet, has been dried carefully. Grey-green dust has been machine-dried hot, and no brewing technique recovers what that cost.
Fermented leaves are a different drink
Bruising a leaf, holding it warm and damp for a day and then drying it is black tea manufacture applied to something that is not tea, and it works. Fireweed becomes ivan chai, which is genuinely dark and malty. Raspberry and blackberry leaf do the same thing less dramatically. Unfermented and fermented material is sold under one name and they are not the same drink — colour tells you which you have.
Three of them contain caffeine
Yerba mate, guayusa and yaupon are all Ilex — hollies — and all three contain caffeine. They are the most common reason someone assumes a herbal infusion is caffeine-free and finds otherwise. TeaHQ publishes no caffeine figures for any of them, because no source it holds carries the five-part basis a figure needs.
Fresh where it grows
Several tropical leaves — curry leaf, makrut lime leaf, guava leaf — are used fresh in the places that drink them, and lose most of their aroma within days. Imported dried material is a much flatter product than the tradition uses, and freezing preserves them far better than drying does. This is worth knowing before concluding you dislike them.