I think the biggest category of rejected ballots are those arriving after 7pm election day.
From recent primary: “More than 35,000 mail-in ballots were rejected in Florida primary”
https://www.politico.com/states/florida/story/2020/09/17/more-than-35-000-mail-in-ballots-were-rejected-in-florida-primary-1317327
If I can find the number of rejected VBM from 2016 GE I’ll post. I thought I found that number on Daniel Smith’s site or twitter but a bit fuzzy.
Found number for 2018 GE. 31,969 VBM not counted.
“In Florida, notwithstanding opportunities for voters to cure missing or mismatched signatures, tens of thousands of on-time VBM ballots are still rejected every election. In the 2018 general election, for example, more than 1/100 VBM ballots received by local election officials were ultimately not counted, amounting to some 31,969 uncounted ballots.”
https://www.liebertpub.com/doi/full/10.1089/elj.2020.0658
Its a tough read. Trust me, you won’t want to.