Yes, it can hurt. First of all, you never know what is in an herbal medicine. There could be too much of the active ingredient, not enough, or none at all. Also, it is very common to have a dietary supplement to actually work, only to be discovered that it was spiked with the actual drug. This has happened with Viagra and supposedly steroid-free psoriasis meds. And plenty of other cases. I follow this rather closely, and virtually every time a supplement is actually studied in a well designed trial it is found to be useless (or harmful—Google news hydroxycut). Add to this the fact that something coming from a plant is no more likely to be safe than something coming from a lab. In fact, the most poisonous substances on earth come from natural sources. Think about it- the plant have evolved not to be eaten in most cases. Thus the alkaloid toxins.
First of all, that' not really true. Herbs are mostly used for sub-clinical pathologies so it is not necessary for the dose to be measured to such a precise degree. If it's the right plant it will most likely have enough of the right phytochemical to have the intended effect. By the very nature of the way plant medicines are used a little more or a little less makes little difference.
It is unlikely that any plant, reasonably collected and processed, will have no active ingredient. In addition to that, dose critical herbs can be found with tested quantities of active ingredient. That only matter for about a half dozen popular herbs. Pharmaceutical drugs are dosed at or near the toxic dose level, generally speaking, because that is how they work. Herbs are only rarely used at the toxic dose level and you won't find those in a store.
Secondly, herbs processed into formulas, like Chinese Patent Formulas, are concentrated and subject to rigorous processing techniques.
As far as contamination, there are a a few rare examples of poor production techniques and facilities. Pharmaceuticals kill and injure tens of thousands of people every year. Very few people are seriously harmed by herbs.
Think about it- the plant have evolved not to be eaten in most cases. Thus the alkaloid toxins.
That is exactly what makes the majority of plant medicines useful. A specific physiological reaction to a plant toxin produces an effect that is beneficial to the ailing patient whereas it would be an irritant or nuisance effect for a healthy person. Such as a diuretic or expectorant. But there are a good number of phytochemicals that produce more complex effects and are not alkaloids.
Using allopathic criterions to judge the use and effectiveness of whole plant medicines works about as well as doing the reverse. It doesn't work well.