I just picked out on citation as given in the Heritage Foundation link:
“In 2011 in Troy, New York, a local political operative was convicted for submitting fraudulent absentee ballots”
The case involved 8 actual ballots alleged to have been illegally tampered with. The case resulted in one single conviction:
Democrat admits role in voter fraud case
THERE IS NO SIGNIFICANT EVIDENCE OF MAIL BALLOT FRAUD, PERIOD. NO DIFF THAN ANY OTHER VOTER FRAUD, WHICH IS ALSO INSIGNIFICANT.
MOST OF THE CASES OF MAIL BALLOT HARVESTING WERE LEGAL, AFTER WHICH LAWS WERE CHANGED. AFFECTED VOTERS WERE NOT COMPLAINING. THEY WANTED THE HELP IN VOTE SELECTION. IT WAS LEGAL.