The argument against it is soaking the health care system for unlimited funds until it collapses or simply becomes unaffordable.
For the horrible things at the edge of treatability and beyond there are always many promising ideas most of them worthless or bad and the price people are willing to pay is either all their money or all everyone's money depending on whether insurance covers it.
If you don't allow insurance to cover it then only a handful of rich folks get soaked but this is politically difficult to maintain.
Insurance already has plenty of leeway to deny coverage for medications, even ones that are FDA approved. If the political unpopularity mattered that much, I'd expect it to already be the case.
Despite the thorny nature of insurance they do already cover lots of things that don't do much good or aren't very efficient. Also medicaid/medicare are more susceptible to political considerations.
For the horrible things at the edge of treatability and beyond there are always many promising ideas most of them worthless or bad and the price people are willing to pay is either all their money or all everyone's money depending on whether insurance covers it.
If you don't allow insurance to cover it then only a handful of rich folks get soaked but this is politically difficult to maintain.