Although the U.S. Supreme Court definitively ruled that the federal government has the power under the Necessary and Proper Clause to keep "sexually dangerous persons" in custody after the expiration of their sentence, both the majority and dissenting opinions noted some issues left unresolved by the decision. From Justice Breyer's opinion:
In resolving [the question presented], we assume, but we do not decide, that other provisions of the Constitution -- such as the Due Process Clause -- do not prohibit civil commitment in these circumstances. Cf. Hendricks, 521 U.S. 346; Addington v. Texas, 441 U.S. 418 (1979). In other words, we assume for argument's sake that the Federal Constitution would permit a State to enact this statute, and we ask solely whether the Federal Government, exercising its enumerated powers, may enact such a statute as well. On that assumption, we conclude that the Constitution grants Congress legislative power sufficient to enact § 4248.
This seems to me an odd way to frame the argument, since it is established that states can act in ways in which the federal government cannot. Justice Thomas' dissenting opinion also mentions the many questions left unanswered by the Comstock decision, although he choose to articulate the questions in laundry list fashion. From Justice Thomas' opinion:
The Court’s newly minted test cannot be reconciled with the [Necessary and Proper] Clause’s plain text or with two centuries of our precedents interpreting it. It also raises more questions than it answers. Must each of the five considerations exist before the Court sustains future federal legislation as proper exercises of Congress’ Necessary and Proper Clause authority? What if the facts of a given case support a finding of only four considerations? Or three? And if three or four will suffice, which three or four are imperative? At a minimum, this shift from the two-step McCulloch framework to this five-consideration approach warrants an explanation as to why McCulloch is no longer good enough and which of the five considerations will bear the most weight in future cases, assuming some number less than five suffices. (Or, if not, why all five are required.) The Court provides no answers to these questions.
Perhaps this is an inevitable consequence of a major decision. In the oral arguments for District of Columbia v. Heller for example, (which also left many questions unanswered) Chief Justice Roberts appeared in favor of having these questions answered gradually over time. From the oral argument transcript:
CHIEF JUSTICE ROBERTS: Well, these various phrases under the different standards that are proposed, "compelling interest," "significant interest," "narrowly tailored," none of them appear in the Constitution; and I wonder why in this case we have to articulate an all-encompassing standard. . . I'm not sure why we have to articulate some very intricate standard. I mean, these standards that apply in the First Amendment just kind of developed over the years as sort of baggage that the First Amendment picked up. But I don't know why when we are starting afresh, we would try to articulate a whole standard that would apply in every case?