Depends on how they're using it. To find an unknown person and prove they were at a scene - yeah you'll have the 100 person's worth of DNA to sort through and then match against a (presently) incomplete DNA database. But if you already have a suspect and need to place them at the scene, if their DNA is one of the 100 then they have shown that.
But we’re they at the scene or did they just bump into someone or something that was there?
That's something that would have to be addressed at the trial by the defense attorney raising challenges.
If the DNA is present, it's present - barring any procedural mistakes by the forensics technicians (mislabeled sample, dirty lab equipment, didn't follow manufacturers instructions, etc). Or deceit by one or more members of the forensics team to implicate the suspect.