My take on all of this...
Lots of states are pushing for larger scale vote-by-mail than has ever happened before. Some states have been doing this for a long time (Oregon, Colorado), but they have the processes in place to handle elections. Other states are trying to do it in a few months with no real prep ahead of time. We're already seeing this lead to ballot printing problems, ex:[0].
To build on that, many of these states aren't allowed to even open/proces absentee/vote-by-mail ballots until election day or after[0.5]. That means that problems with these ballots may not be caught ahead of time. Along with that, having to processing millions of ballets (when they are used to 5-digit amounts of ballots), means it will take a lot longer to handle those ballots. This will lead to a delay in knowing the results. As well, mail-in-ballots get rejected at a much higher rate than in-person voting[1], which will lead to lawyers fighting over which ballots are valid if things are close.
Democrats are more likely to vote-by-mail than Republicans[2]. This means those delayed ballots will likely swing towards Biden, which means as days go on for counting, Biden's lead is likely to increase (which is totally fine, but it will definitely stir up controversy).
Then you have lawmakers and judges changes the rules[3]. There are a few cases going on around the country where there is a push to allow late mail ballots to be counted. In this case, a Judge decided to allow ballots to arrive up to 3-days post election. These kinds of cases are going on around the country and still being fought.
It's going to be messy.
[0] https://www.washingtonpost.com/local/va-politics/virginia-vo...
[0.5] https://www.ncsl.org/research/elections-and-campaigns/vopp-t...
[1] https://www.npr.org/2020/08/22/904693468/more-than-550-000-p...
[2] https://www.usatoday.com/story/news/politics/elections/2020/...
[3] https://nymag.com/intelligencer/2020/09/pennsylvania-becomes...