A couple of states have had vote-by-mail for years, and Oregon at least has had it exclusively for decades.
The Republican Secretary of State ran an election audit in 2016 and found that, over 2+ million votes cast, around 50 or so were "fraudulent," most of those being cast by people who voted in Oregon and elsewhere.
Vote-by-mail is not problematic, if you consider the historical evidence provided by places that have been doing it for a while.
That is the party line, but you have no way of actually knowing that. The election audit to which you refer has no way of knowing how many votes were cast by identities that were simply invented because of the ease of registration in many states (which has now been combined with proactive sending of mail-in ballots, thanks to COVID). Sure, they can tie together people who voted in more than one state, but that isn’t the attack vector that anyone with any sense would use. Show me a good way of identifying brand new identities used to vote in states where there are easy ways to bypass ID requirements (such as California). There isn’t.
Vote-by-mail allows pressure tactics and potentially removes the privacy of voting. For example, abusive spouses can control the ballot of their victim. It also allows payments for votes, because the ballot can be checked by the payer.
The Republican Secretary of State ran an election audit in 2016 and found that, over 2+ million votes cast, around 50 or so were "fraudulent," most of those being cast by people who voted in Oregon and elsewhere.
Vote-by-mail is not problematic, if you consider the historical evidence provided by places that have been doing it for a while.