DNA evidence is overrated. People leave their DNA everywhere, so it's not that hard to get some and then plant it somewhere else.

The tests also have varying accuracy rates, but people misunderstand what it means. If the test is 99.99% accurate, that doesn't mean that there is a 99.99% chance that the defendant is the perpetrator. It means that in a region with ten million people, you've whittled your suspect list down to a thousand people. If you pick one of them at random there is only a tenth of a percent chance it was them.

This especially problematic when dealing with "DNA databases" because then with a large database you have a high probability of finding a false positive match and the true perpetrator might not even be in the database.