I have lived in both. IMO, the universal mail-in ballot is great in many ways, and please don't forget that the current regime of 2020 election deniers equates "voter fraud" with "We disagree with the results".

Two paradigms apply to the cult:

1. Every accusation is a confession.

2. An entire group is defined by a designated outlier. (Think "Willie Horton")

But technical issues of very late vote count are above my pay level to diagnose and fix, even to what extent any fixing is worthwhile. Think Pareto Principle. You want to fix the 20% of causes that lead to 80% of the problem. Both neglect and perfectionism are common enemies.

Many of the flaws are self-imposed. The requirement for ballots to merely be postmarked by election day is insane. If my credit card bill is due on June 5, it's due on the 5th, not postmarked by. I will pay penalties if it arrives past that date.

If you have the privilege of voting by mail then be an adult and mail it promptly so it arrives by election day when all ballots should be counted. Ballots arriving after should be summarily rejected.

California also allows same-day registration, another insane innovation ripe for irregularities.

A compromise would be vote-from-home but dropoff-in-person (where ID is checked). I would argue most states allow ample time for early voting, sometimes weeks ahead of time, allowing just about anybody to fit it into their schedule to vote in person. The arguments for default vote by mail (barring some verified hardship) simply don't hold up enough to offset the potential negatives.