#1 reason would be the heart issues. My heart actually hurt on clen.
It also has damaged thyroid levels permanently for many guys.
Not sure what else would be worse than these two. Killing your liver with orals would be worse, but at least you have a warning, and you can guarantee it won't happen with the right doses. T3 can also hurt the thyroid, but that's another dose-dependent thing.
Those are the reasons why it's dangerous. Another reason I don't like it is that it killed my workouts. Can't make strength gains at all on clen. Albuterol I can for sure. It burns off very little fat compared to what some people hype it up to do. I remember when I first started bodybuilding that guys would say they could eat whatever they wanted and the clen would just take all the fat off...hell no lol.