I'd argue we need evidence that there was a fair election, not evidence that there was fraud. If there's non-trivial ambiguity, there's room for fraud, and people will likely enough exploit it.

Stupid example: What if only Republicans were allowed to count ballots, and the only result you got was a number of votes that they told you, and they threw away all of the records. It would be impossible to get evidence, but you wouldn't need any evidence to rightfully distrust the result.

Purely electronic voting systems are analogous, though it differs in the details and degrees of things.