I've heard that anecdotally too, and tbh I would never ingest something a stranger sent me in the mail. I'm sure it's mostly fine, but there's a level of trust that whisper networks provide that is simply absent in an online setting. Heck, bitcoin itself purports to do exchanges in a trustless setting; trustless is the last environment you want to have for things you're putting in your body.
I've been buying drugs on the street and online for many years and online crushes IRL, just like it does for legal commerce. Online reviews have helped me always get top notch quality and exactly the product I expect. The street can't come close.
Actually darknet drug sites do have a review system, it's not like people just order randomly.