I think another disadvantage that the post doesn’t mention is that being an Organization confers a sense of legitimacy, in part because it acts as a costly signal that one can manage all the legal complexities, pay for paperwork, etc.; so being a sponsored project is a way to call yourself an Organization without paying the cost of the signal. In the limit of widespread knowledge about the legal subtleties of fiscal sponsorship, and due diligence on the part of the community to always keep the distinction in mind, this is not a problem. But people are lazy, and so this sort of confusion does I think benefit sponsored projects.
I think another disadvantage that the post doesn’t mention is that being an Organization confers a sense of legitimacy, in part because it acts as a costly signal that one can manage all the legal complexities, pay for paperwork, etc.; so being a sponsored project is a way to call yourself an Organization without paying the cost of the signal. In the limit of widespread knowledge about the legal subtleties of fiscal sponsorship, and due diligence on the part of the community to always keep the distinction in mind, this is not a problem. But people are lazy, and so this sort of confusion does I think benefit sponsored projects.