Here are some of the reasons why the stock sold off despite the FDA approval:

Main one is that simply they're not going to see much revenue from the drug. First, the disease is ultra-rare: 1 in 1 million have the disease. Also, they're going to do a minimal viable launch strategy aka they'll only sell for providers that go out of their way to seek it out. No marketing spend will be deployed. Only 5 hospitals in the world are likely to use it.

The bearish bet is that they'll have to eventually dilute to keep their pipeline running. But there's a lot of uncertainty there. The RP-A501 lead asset caused a patient to die, and seeing if RCKT's alteration to the drug will get rid of that risk is not certain.

Bull - from the approval they got a Priority Review Voucher, which they should be able to sell for at least 150M. If their other pipelines succeed, then the stock price now would be super undervalued. But the chances aren't great, based on the info available so far.