In the book Botany at the Bar: The Art and Science of Making Bitters by Selena Ahmed, Rachel Meyer, and Ashley DuVal, the authors list some ingredients that they recommend against using. The authors work in sustainable food systems, conservation genomics, and plant genetics in university settings, so they're qualified to comment on this important topic. 

"We do not use the following botanicals commonly used in traditional home remedies for preparing alcoholic bitters."

The list is:

• Arnica
• Creosote/chaparral
• Ginko leaf
• Kava kava
• St. John's wort
• Birthwort
• Digitalis
• Madder root
• Senna
• Wood fern 

While these ingredients probably deserve a more thorough write-up as to why they are specifically disrecommended in alcoholic infusions, for now this page will serve as a placeholder and warning.