No risk. Many states have been doing it for years with no problems.
Too much room for doubt.
We had a mayoral election where a candidate was ahead w/ votes from the voting booths. A van load of empty ballots were placed in the same room as the mail in ballots in the middle of the night by a campaign worker of the losing candidate.
What could go wrong? Luckily the winning candidate had workers placed in a clandestine location to observe the impropriety, the campaign worker was detained , the incident was reported, police showed up and the empty ballots were removed.
With mail in ballots the results of the polling places can be counted and since the mail in ballots can be counted for days, weeks later empty ballots can be used to make up the difference.
I have absolutely no trust in an election where mail in ballots (not the absentee ones which are far more vetted) are in use. Never have, even years before the 2020 election. The positives outweigh the negatives in those situations.
We've used them for a long time in muni elections where there's lengthy bond issues. They're also good for being able to look at the record of judges who are on the ballot.
I remember the first time mail in ballots were used for this purpose my first thought was that , well this is OK for a muni election where one needs the time to examine the pros and cons of bond issues or judges' records, but god help us is these are ever used for a statewide election or national election of candidates where most of our minds have been made up.
I'm not saying the 2020 election was rigged, but I
am saying it was very, very flawed.