The problem is that the in the US the right wing has claimed there was fraud with mail in voting. This means that if anyone mentions the issues with mail in voting they are assumed of doing the same thing.
No matter whether you are left or right, mail in (or god forbid online voting) is an horribly bad idea. The whole voting process should be transparent and visible from casting your vote (or someone elses) to seeing it counted. That is not possible with mail in voting so it allows for possible fraud (or even the suggestion of fraud as we see in the US).
Someone mentioned unique barcodes on ballots, great now you have eliminated voting anonymity.
Yes, voter suppression is a real issue mail in voting can help against, but IMO mandatory voting is the better solution.
It’s still much less transparent with more possibilities of things going wrong and it doesn’t guarantee a vote was done anonymous.
Mandatory voting is the much simpler solution