What wouldn't be traceable in jurisdictions where they do not perform signature verification is ballot swapping (or even if they do since signatures are easy to duplicate and judges have thrown out signature verification in some jurisdictions anyways). Which can happen at any point along the way. You mail off your ballot and then you might be able to check if it was received and recorded. But you can't see what votes were actually recorded for you. So a man-in-the-middle attack can modify that ballot and no one would know. And there are so many opportunities to do so, even if you're not talking about ballot harvesting.
The problem is that the only way currently to verify the above is to canvas every household that mailed-in and verify the vote that was recorded was actually the vote that they intended to file. That remedy was denied by nearly every judge along the way. What we end up with in real life is that there is no way in current law to detect and remedy a competent man-in-the-middle attack in a Presidential election because of the tight timeframes enacted.
It doesn't help that there have been videos of Democrats around the country admitting to doing exactly what was described and they are neither investigated nor are they carried by the MSM. This kind of sweeping-under-the-carpet causes people to have distrust in a system that is obviously flawed.