First and foremost,
a partisan split has already occurred.
In North Carolina, 643,400 voters have requested an absentee ballot as of Sept. 3. Of those requests, 337,362 were made by registered Democrats and 103,620 by registered Republicans.
Which leads us to:
The Nightmare Scenario That Keeps Election Lawyers Up At Night
A “red mirage” on election night and a post-election “blue shift” could lead to an unprecedented constitutional crisis just as the Supreme Court is in turmoil.
Second, the rejection rate of mailed in ballots will be higher than Trump's margin of victory from 2016.
Tens of thousands of mail ballots have been tossed out in this year’s primaries. What will happen in November?
There is a demographic bias in ballot rejection. Think you might want to challenge which ballots get counted or tossed out?
Still think the SCOTUS will not be called upon to decide 2020? Bush V Gore is nothing by comparison.
Bonus:
Do not open until Election Day: State laws will delay counting mail-in votes
14 States do not even authenticate the ballots until after the deadline. Let alone count them. Leading to a big post-election fight.