I will assume you are referring to crime scene testing in criminal cases. DNA samples taken from the scene of a murder, rape or other crime can be used to confirm the identity of the perpetrator through comparisons with DNA taken from the suspects. It can also rule out suspects whose samples do not match.
DNA analysis is highly accurate. Even very small quantities can be copied multiple times in a procedure called PCR (Polymerase Chain Reaction) so that there is enough to analyze. When the samples are run through gel electrophoresis, banding patterns can be verified with over a 99% accuracy rate, generally assuring that only one in many millions of people would match.
There are rigorous rules to ensure the sample is not contaminated. Samples must be sealed and the chain of custody must be documented. Because it narrows down suspects with such a high degree of accuracy and can be gathered from so many bodily sources, there are mostly pros associated with DNA evidence. Cons would mostly come into play with mishandling of samples. There can be issues if a person tested has recently received a blood transfusion, but if this is known, corrections can be made to account for the foreign DNA.