There are a few downsides to DNA evidence:
It has been suggested that the prominence of DNA evidence on TV
shows has caused juries to expect irrefutable DNA evidence before
convicting someone. This may be a problem if other forms of
evidence are ignored.
DNA evidence can only be obtained in instances where biological
substances are left behind or exchanged. This only occurs in a
minority of cases.
DNA evidence sometimes only proves that the person was present
at the scene - it does not always prove guilt. However, if a
person's DNA is found at the scene, this may be perceived as proof
that they committed the crime.