The limitations an investigative forensics team encounters upon trying to identify 20 year old remains depends on the climate and conditions in which the body was found.  The human body can take anywhere from three weeks to a year in temperate climate conditions to completely degenerate to the skeleton.  In tropical climates, the process is greatly accentuated, resulting in a skeleton in about a week's time.  Another factor would be whether the skeleton has been scattered, as a result of wild animals, or centrally located, with all the bones intact.  Even the skeletal remains, given a 20 year time frame and the acidity of the soil they are located in, could eventually be dissolved into the soil, leaving little to nothing for investigators to go by except teeth for dental records and skeletal DNA.