The voting machines that americans are talking about [1] aren't "fully electronic black boxes", they're "paper ballots + a vote counting machine + maybe a machine that acts as an overly fancy pencil (but let's you verify the paper ballot afterwards, i.e. what you talk about in your last paragraph).
The paper audit trail, physical ballot box, etc still exists.
I don't know if these machines exactly justify their cost... but they don't seem to be security risks.
[1] So by this I mean "only the republicans are complaining right now" and "they aren't complaining about states where non-paper voting machines used". In fact it looks like that of the 7 states with some form of non-paper voting machines, all but New Jersey went for Trump. New Jersey was never going to be close enough that the republicans feel like they can complain about it.
[2] List of states and how they use voting machines here: https://ballotpedia.org/Voting_methods_and_equipment_by_stat...
Kentucky, Lousiana, Mississippi, New Jersey, Oklahoma, Tenessee and Texas are the ones with "DRE without VVPAT" (voting machines that are more than a fancy pencil and/or paper ballot counter).
[3] Election results from https://en.wikipedia.org/wiki/2020_United_States_presidentia...