That would depend upon how they were done. It is trivial to print a ballot that is both machine and human readable for voter verification.
You might want to check out some of the other stuff Mr. Schneier has written about electronic ballot (and other) issues. The Crypto-gram is required reading for me.
You're right about the dependence on implementation/operation.
Elections shouldn't depend on whether the right people are conducting them or whether they are doing the job correctly.
The best systems are designed to be failsafe even under the most adverse anticipated conditions.