The phrase "thorny subject" means a topic or issue that is difficult, sensitive, or that may cause disagreements or problems when talked about. Full definition
Stay tuned till next time, when we'll tackle the even thornier subject of peanut butter: «creamy or chunky?» (edery.org)
Why does swapping mindless pleasantries for thornier subjects seem to have such a large effect on people's mood? (businessinsider.com)
The condition, named post-treatment Lyme disease syndrome, or PTLDS, is commonly known as «chronic Lyme disease,» and on the Internet, it's a notoriously thorny subject. (sciencenews.org)