* I was raised in a country with a single party and 99% outcome elections. People, kids my age (at the time), sacrificed their lives for the right to vote. I don't take voting for granted, but I do take voting seriously.
* The mail-in argument is predicated on the postmodern "the exception is the rule". I reject that on principle. A small minority has special needs, we can find many ways to accommodate that without disrupting the vast majority. And if we can't, too bad for the small minority.
* The custody chain is fundamentally broken. The reason I marginally trust election results is because representatives of both parties keep a hawk eye on each other to prevent foul play. This can be done in at in-person polling locations for a day or two, but cannot logistically be done for weeks over the area of an entire state.
* As a corollary, him who delivers the mail has weeks to either make certain votes appear from thin air, or disappear / delay blocks of votes from areas likely to vote against his party. Oops, we just printed 10k ballots too many...
* The vote must be secret. Mail-in vote sits with your signature on it for weeks who knows where. In the era of cancel culture, possibly worse, the doxxing risk is non-negligible. Remember Brendan Eich?
* There is no confirmation that the vote was filled by the person receiving the ballot. Perhaps their spouse / elder guardian / etc. filled it in for them?
* Ballot harvesting, the process of pressuring people into voting by knocking on their door or downright bullying them into collecting the blank ballots. Done selectively in areas leaning towards your party it can easily turn an election. Note how both parties complain about this when they are at the receiving end of it.