Waste of Effort
Electronic voting systems are a solution to the wrong problem.
You can introduce various layers of complexity, audit trails and so forth, but none of this changes the fact that most people would be incapable of understanding the operation of electronic vote-counting equipment even if the complete blueprints, schematics and software listings were publicly available (which ought to be a minimum requirement for consideration anyway: nobody's "intellectual property" is more important than the very foundation of democracy). And how can you reasonably expect someone to trust what they do not understand? Whereas pencil and paper, ballot boxes and manual counting have the property of being universally comprehensible.
If there is even one layer of abstraction between the physical action taken by the voter and what actually gets counted, what's to prevent them from claiming some utterly bogus totals as the "final count"? Just because you know how you voted, that doesn't prove anything about how your vote was recorded. You could have 500 people voting for A, 480 people voting for B and 120 people voting for C; but the totals could be *claimed* as 390 for A, 600 for B and 110 for C. And nobody would be any the wiser, because the errors are just about within the bounds of plausibility (only a sample was surveyed, and people often lie to reporters about who they have just voted for / are going to vote for). To discover the error, you would have to check everybody's votes and see that the final totals are not as claimed; and you'd be arrested for Treason long before you got far enough to notice any anomaly.
Even including some kind of hand-countable paper ballot "to check how you voted, and as a backup in case things go wrong" is a misguided strategy. (At the very least, it means admitting that things can go wrong.) How *you* voted is irrelevant. It's how *everyone else* voted that swung the election. Seeing something that proves you voted for candidate A is meaningless. You don't have any way of knowing who the 499 others who really voted for A are. As far as you are concerned, you and 389 anonymous strangers voted for A.
Your paper ballot which you drop in the backup box may match the record of your vote (which can be checked by you or anyone seeking to know how you voted, but *doesn't* necessarily contribute to the running total for the candidate you thought you were voting for); but if those paper ballots aren't going to be counted by hand (because there is a plausible alternative explanation for the discrepancy, i.e. "That is how the people actually voted, and it must be true because the machine says so") then what's the point in having them?
If the paper ballots are going to be counted by hand (which will reveal that the election was rigged somehow, but the Powers That Be have all manner of techniques at their disposal to prevent that happening), then what was the point in having all that expensive machinery, when you could just have used paper ballots and hand-counting in the first place?
And if the paper ballots are going to be counted by machine, then the counting machine can always be rigged to produce totals which do not reflect reality: 500 in the "A" pile under a display reading "390", 480 in the "B" pile under a display reading "600" and 120 in the "C" pile under a display reading "110". If the ballots are deliberately "mistreated" by the machine, so as to give them a slight but randomly-variable curvature, then the discrepancy in sizes of the piles won't be so apparent.
It is reasonable to assume that a hand count is certain to be correct, since an adversarial relationship exists between the candidates in an election. That is to say, candidate A and their aides do not trust candidates B, C or their respective aides; candidate B + aides do not trust candidates A, C or their aides; and candidate C + aides don't trust candidates A, B or their aides. As long as the actual candidates and/or their aides do the counting themselves, and all sides are represented, everyone will be watching everyone else like a hawk and there will be a strong disincentive to cheat.