If you're talking about the NY primary, it didn't have much to do with the mail, it was mostly people who were unfamiliar with the process and mixed up things like forgetting to sign it.
Patel said his campaign is looking at thousands of other votes that seem to be missing postmarks entirely—a post-office error... He sent me a photo of a box of 3,000 absentee ballots from just one part of the district that have been ruled invalid because they lack a postmark.
Personally I've heard a lot about the mail being an issue in NY.
Here's an atlantic article about it: https://www.theatlantic.com/politics/archive/2020/07/new-yor...
That said: I think New York is an example of why the debate about this is worth having. 20% of ballots getting tossed is catastrophic.