That is still an overly broad category to be worried about in general... just because something is synthetic doesn't mean it is more or less dangerous than a non-synthetic thing.
Non-naturally occurring chemicals, or at least chemicals that we weren't traditionally exposed to in great abundance. Like laundry detergent and conventional shampoo. Those are probably terrible for the environment. When we ban one, companies just create another one.
Toxicity is one measure. What about endocrine disruption? What ability long-term effects like cancer? And that's just for humans. We don't know how some of these compounds that end up in our waterways are affecting marine line.