In one of those mind jumps from one thing to another, I thought of something a little similar. The chain of evidence thing, and how much or little proof the testing labs can show that the samples are uncontaminated and are definitely tied to the person who claims to have submitted the sample. For starters, they can't make any claims whatsoever to the validity of the collection process - simply not present.
It would seem a moderately sharp defense lawyer could get the whole connection trashed on grounds that the Ancestry (or other) DNA process does not meet criminal lab control; and that even if that DNA analysis could make someone a suspect, it is nothing more than hearsay and can't even be used to force someone to submit a legally recognized sample. Perhaps even to try and use it as a reason to take a look at someone was the same as profiling.
Since these DNA labs stand to lose money, it would stand to reason that they themselves would make a strong claim that their process, while maybe good, doesn't and was never intended to meet the standards used in the criminal justice realm. And then there's the legal lottery thing - somebody "harassed" by the cops suing the hell out of Ancestry or whoever for releasing information in ways never intended.