Forensic scientists must deal with heavy caseloads, perpetual backlogs, and tight deadlines, which can lead to stress and burnout.
It's incredibly rare for a laboratory to have enough resources to conduct the work they’re asked to complete. For tax-funded government laboratories that can't charge for services, there's no incentive for 'customers' to be judicious in what they want done. "Test everything" may seem like the best policy from that stakeholder perspective, but is not sustainable for a lab in the middle- and long-term.
The pressure to complete work quickly must be balanced with limits on what is accepted for testing. If this balance is disrupted, labs can become crippled with massive backlogs and the inability to keep up. On the other side of the balance, police, attorneys, and judges need to know how the evidence from their case can inform their decisions in order to try the case properly. It’s common for labs to implement submission policies or guidelines which limit the number and/or types of samples accepted for each test.
Overall, these issues create tension in the system; stakeholders who at least think they know what they need tested, and laboratories who know they can’t do everything for everyone without failing. The laboratory also understands that ‘failure’ in their high-volume and high-stakes environment will mean re-testing of evidence, Corrective Action, internal investigations, and/or improper conviction of the innocent or release of the guilty.