I don't have experience with mail-in ballots, but ton of experience with provisional ballots. Over the four years that I worked the polls, I would say a fairly large percentage of those turned in with the assistance of poll workers were spoiled. And because of some boneheaded errors: writing in today's date instead of your date of birth; putting down the wrong address; not signing the ballot; using a ballot for the wrong party (someone switched parties but was not yet eligible to vote for the new party's primary candidates), etc.
It will be a lot worst with no assistance and the extra security measures in place for mail-in ballots. Especially for states that don't usually allow easy access to MIBs. And then add on the USPS head honcho's shenanigans. It's going to be a real show.