While simple heuristics like that can get you quite far in life, it doesn't mean that they are laws of nature. I suppose time will ultimately tell, but these drugs are super widely used and apparently the class (GLP1 agonists) is not as novel as you might think; the blockbusters we see today are improved versions of drugs that have existed for awhile. If there was some horrible, monkey's paw catch, we'd likely have seen it by now.

I suppose the biggest example of such a faustian weight-loss drug would be Fen-Phen, and it's issues were discovered within a few years of its rise to prominence.