IANAL but my understanding is that there are specific criteria for what is admissible and what isn't. For example, there are very specific heuristics for determining whether something is hearsay. What I'm saying is that there should be a clear cut list of criteria regarding which circumstances are admissible (e.g. was the confession made freely or was there coercion/promises/negotiations/etc involved, was the conversation recorded/unadulterated, etc) and that this list should be properly followed to disqualify potential foul play, just as is done w/ hearsay.