Reliability
On one hand, confessions obtained under torture have often been considered to be not objective enough, since the use of such means may lead to the suspect in confessing anything. However, when the confession reveal secret only known to the perpetrator (such as the location of the body or murder weapon), the confession is reliable.
On the other hand, even without torture, various cases of avered false confessions demonstrate that, in itself, one man's confession is not a sufficient proof. False memory (including memory biases, etc.) or privileges granted under plea bargaining might lead to such false confessions.
In Japan, the legal requirements dictate that confession is only admissible as evidence only if it contains elements only the guilty could have known. However, many miscarriage of justice cases in Japan are due to the police faking the confession of guilty secrets.
Read more about this topic: Confession (law)