So each individual allele frequency varies depending on the population, but when we take a collection of say ten alleles, the ups and downs tend to cancel out. We find that there is not really a great deal of difference in the profiles we have seen from one group to another. When we modify the product rule appropriately by measures of population substructure, then the ceiling principle is inappropriate. I would think the weak link would be right at the beginning, for example, does the tube labeled 'crime scene blood stain' reflect the true source of that material. The forensic laboratories have a lot of safeguards built in, such as dual observation of each step, and signing for custody of the evidence.
Forensic laboratories have a lot of experience in taking care with evidence.
But I take your point, if there is going to be an error, it would be of the gross human kind, rather than in technique. One criticism of forensic DNA profiling as opposed to genetic susceptibility testing, is that the methodologies have not passed through the usual channels of peer review and comment. Some people were very jealous about these data. However, the data are now routinely made available to researchers. There is now an extensive bibliography of studies available.