From "Law and Order" to "CSI," not to mention real life, investigators have used fingerprints as the gold standard for linking criminals to a crime. But if a perpetrator leaves prints from different fingers in two different crime scenes, these scenes are very difficult to link, and the trace can go cold.
And even that one is useless in case of identical twins.
And it’s easy to get a contaminated sample. There have been plenty of convictions based on DNA that have been overturned.