It's because of this technique that many offenders from the past are being brought to justice. However due to some recent cases it will be harder to get a conviction as the lawyers will stress the unreliability of the method. Unless that is the person brought to justice feels it a done deal and admits to the crime.
It very much depends on how the 'evidence' has been stored. The discrepancies start when the available DNA is magnified for profiling, any slight damage is also profiled has has been proven to point to an innocent person.
A major problem with the classical systems that test proteins found in blood, is that very few of these proteins are also found in semen and other body fluids. Because the majority of cases that require genetic typing are sexual assaults, the lack of a definitive set of useful genetic markers in semen has long been a great handicap to the scientific analysis .
Semen stains can effectively be tested for DNA up to 7 yrs and even cases in up to 25 years.