If you grow the herbs, be meticulous about keeping different plants separate when you harvest and dry them, and obsessive about labeling. A simple is one herb. The more herbs there are in a formula, the more likelihood there is of unwanted side effects. Understandably, the public seeks combinations, hoping to get more for less. But combining herbs with the same properties, such as goldenseal and echinacea, is counter-productive and more likely to cause trouble than a simple.
Different people have different reactions to substances, whether drugs, foods, or herbs.
When herbs are mixed together in a formula and someone taking it has distressing side effects, there is no way to determine which herb is the cause. With simples, it's easy to tell which herb is doing what. If there's an adverse reaction, other herbs with similar properties can be tried. Side effects from herbs are less common than side effects from drugs and usually less severe. If an herb disturbs the digestion, it may be that the body is learning to process it.