Not so much that supplements can be harmful physically, more so they create a false sense of health and allow us to ignore the things that matter. For example, say someone has psychological issues, and they just take a pill to stabilize their mood, they are now relying on that rather than addressing the issue.
Say Joe bodybuilder is taking 5g a week and is pushing 220lbs and his blood pressure is 160/95. Then he finds the perfect combination of supplements, and his blood pressure is now 130/70. Did he really solve his problem or is it being masked? If he didn't think he found the magic supplement combo, maybe he would have decided to cut down to 205lbs (less stress on his heart), drop his cycle down to 2g a week, clean up his diet a bit, and start doing cardio 5x a week.
But he was Able to find a combo of pills to check his blood pressure she he continued to abuse his body thinking it was ok.
Thats my main point. We want to look for a pill or powder to make us healthier that allows us to continue to do the things the made us unhealthy in the first place. I see alot of threads about supplements that claim to help the kidneys, those are the ones that I find the most dangerous. If someone is abusing gear, pushing bodyweight, pushing blood pressure to a point where the kidneys are compromised, they should really re-evalute their lifestyle. Thinking they found a herb that will save their kidneys and allow them to continue as is could be very dangerous IMO.
Another example could be caffeine, maybe the most popular supplement out there. Lots of people take it to get them through the day. But if they need caffeine to stay awake, id argue they are better off addressing the issue of why they are tired all day. Overtraining, working too many hours, not managing time and getting enough sleep, poor diet, too much stress, etc. The caffeine acts as a crutch.
Another one I see is when guys take deca to lube the joints so they can lift pain free. If someone has pain when lifting, say the shoulder for an example, id say they should change their workout programming. They could be over training, or maybe just at age 35 or 40, they just shouldn't be pushing heavy loads anymore and should focus on high rep work before they really tear something. Or maybe they have a legit injury that could he healed completely with surgery or physical therapy. But if they take deca to mask the pain, it could just become worse as they continue to abuse the joint and something could eventually tear.