If democrats can cheat with voter fraud so can republicans, this is where things can get messy.
Now the big problem is going to be mail in voting, this can help democrats or horribly hurt them, reason being republicans seem to favor voting in person while democrats favor voting from their house. This raises a question of how dedicated are democrats to biden if they will not vote for him if they are required to leave their house.
But the bigger question is how many votes will be thrown out, so far I read somewhere today around 500k votes were already thrown out for either being late or improper or for other reasons during the primaries alone, and this does not count them combing through to check for other things. Now imagine this on a nationwide scale, if instead of 500k votes, it ends up being 3 or 4 million votes discarded for being late, or improper, or for whatever reason, that would be enough votes to throw an entire election without cheating, simply by following rules.
Hence democrats and republicans should be scared of large scale mail in votes, ballot stuffing is always a concern, but a greater concern is your vote gets rejected or never arrives to get counted, as the mail system was never intended to handle such, and states can not handle so much in terms of late ballots, and the election results can not be pushed back months because states wanted to implement a mail in system that would take years to get right midway through an election cycle without ever stopping to think about it.