But presumably that will eventually end up in an adversarial court? It's only secret until it's used as evidence. The NSA's targets will likely never get to defend themselves or debate the legality of the collection of evidence.

Your last sentence is pretty key.

It's outrageously difficult for US citizens to bring a case against illegal or questionably-legal domestic use of our foreign intelligence gathering programs, tools, and/or assets.

There is also no secret court that rubberstamps all requests made, no matter how ridiculous.

Search and wiretap warrant requests are secret (until served), and are (especially in the wiretap case) approved the overwhelming majority of the time.