Guantanamo Newspeak
Officials who work on the Guantánamo issue say administration lawyers have become concerned that they would face significant obstacles to trying some terrorism suspects in federal courts. Judges might make it difficult to prosecute detainees who were subjected to brutal treatment or for prosecutors to use hearsay evidence gathered by intelligence agencies.
U.S. May Revive Guantánamo Military Courts – NYTimes.com
I'm sure the government's lawyers trying to prosecute terrorism suspects would face a significant obstacle in federal court: finding 12 jurors who don't hear the word "terrorism" and immediately give the government whatever it's asking for; they might have to go to an NACDL or ACLU conference to find their jury.
But that's not the sort of "obstacle to trying some terrorism suspects" that they're talking about. What they're talking about are obstacles like the Fifth Amendment (which would bar evidence obtained through coercion and require due process), the Sixth Amendment (which might bar intelligence reports and other secondhand evidence, and which would allow a jury trial) and the Rules of Evidence (which would bar hearsay): obstacles not to trying terrorism suspects, but only to convicting them.
The Bill of Rights is indispensable, but the Rules of Evidence are statutory. Congress could conceivably, and perfectly legally, create a tribunal in which some or all of them don't apply.
Such a tribunal would still be governed by constitutional principles. For example, while Article 8 of the Federal Rules of Evidence (dealing with hearsay) might be abrogated, out-of-court testimonial evidence would be inadmissible under the Confrontation Clause (whether intelligence reports are "testimonial" for purposes of the Confrontation Clause would be an interesting question).
But procedural rules in federal criminal court have evolved to provide a rough approximation of a fair trial. Start cutting out rules that make it more difficult to convict, and pretty soon you're not playing fair. (Play unfairly enough, and you bump up against the Constitutional limits of the Due Process Clause.)
Sure, we can build a playing field that would make it easy to convict KSM or a dope dealer or you or me. Just because we can doesn't mean we should.