The Atlantic recently published a very informative article about how DNA testing has become less credible relevant and reliable as its use has grossly expanded based upon inadequate training, misinterpretation, and misuse. It is a must read for anyone interested in the appropriate role of forensic science in criminal justice.
Discussions of contamination, human errror, storage, and flawed analysis issues are within and are important, but just as important is a common sense problem with increased use of DNA to assign guilt in crimes even where everything is done correctly - that is that MANY people leave DNA at crime scenes who did not commit a crime.
For example, everyone who touched a doorknob or object or person will leave trace amounts of so-called touch DNA on a victim or scene for an extended period of time - perhaps days. Identifying someone who had contact with a person or scene does not mean they committed the crime. Another example that often occurs in child abuse cases is that an article of laundry with semen on it in the washing machine will put detectable semen on everything else washed with it.
Thus, DNA, on its own can mean almost nothing and can cause investigators and jurors to fall prey to ignore other exculpatory evidence or their own common sense. And that is everything is done correctly, which it often is not.