I think the precise location where the DNA is found is more important to proving a case than merely finding it. Environmental DNA lacks this information. When collecting DNA from a scene, I believe there are guidelines and really just plain common sense where you'd want to swab.
That is, finding someone's DNA in a common area is less convincing than finding it on the handle of a weapon. If environmental DNA is abused by an overzealous court, it may call into question the general effectiveness of DNA testing.
I remember a case study where they found the accused with DNA, only to find out, while the match was good, the match had been dead for a couple years and couldn't have been the perpetrator.
These "false positives" would make confidence in the system less.
