In the current political climate here in 2020, it's nearly impossible to get an expert unbiased source on this topic.
Actually, there are probably some but its impossible to know if they are unbiased and experts.
So I went back 3 years, to a time before this became so hyper political (Trump in 2020 is concerned about the reliability of mail in voting, that means everybody else from the MSM and dems have to tell us the opposite)
The article below tells us why, no it's not reliable and its not the post office responsible for many of the problems.
Voting by mail grows in popularity – but is it reliable?
In 27 states, voters now can choose to vote by mail. But unlike votes cast in person, many absentee ballots wind up uncounted, for reasons ranging from invalid signatures to simply being late. Experts say the method is also vulnerable to fraud. December 21, 2017
Nationwide, roughly 24 percent of all votes in the 2016 presidential election were cast via absentee ballots. That’s 33 million votes, according to the federal Election Assistance Commission. But what many Americans don’t know is that nearly 400,000 of those ballots were never counted, having been disqualified for reasons ranging from invalid signatures to simply being late.
While 400,000 votes may not make a difference in a landslide, most elections don’t end in a landslide. For example, the 2000 presidential election was decided in Florida by 537 votes. In a close race every vote can make a difference. That is particularly so, election experts say, in down-ballot races at the state and local level during low turnout elections.
Like the 55 ballots in Virginia’s 28th District, absentee ballots sent through the mail depend on the reliability of the US Postal Service. If absentee ballots are lost in the mail or delivered late, they won’t count.
But even if they are received on time, such ballots might be disqualified because the voter forgot to sign the mailing envelope or because officials determine that the voter’s signature wasn’t a close enough match to the signature on file in the voter registration database.
Critics of mail-in voting also raise the specter of fraud.
In-person voter fraud at a polling place is difficult to accomplish and rare, according to election experts. In contrast, absentee voting presents a significantly easier opportunity for someone looking to rig an election.About these ads
“It is true there isn’t yet a good case of large-scale fraud where someone got hold of hundreds of thousands of ballots and then cast them on behalf of people who aren’t the actual voters,” says Professor Gaines. “But it is plainly not a desirable situation to have loose ballots that are unaccounted for, in the sense that they are out there and could be cast.”
Roughly 20 percent of the 41.6 million absentee ballots sent out to voters in the 2016 presidential election were never returned. That’s nearly 8.3 million ballots sloshing around in a sea of junk mail during election season. In addition, 583,000 of those ballots were returned to election officials by the Post Office marked as undeliverable.
“It is just creating huge holes in the system everywhere,” says Gentry Lange, a Seattle real estate broker and director of the No Vote By Mail Project.
With such a large number of un-voted ballots, it is remarkable that there aren’t more absentee ballot fraud cases, some experts say. But there are some.
Among recent examples is one in St. Louis, Mo., where federal agents are investigating suspected fraud related to the 2016 Democratic Primary for the District 78 seat in the state’s General Assembly.
The contest was between incumbent Penny Hubbard and Bruce Franks Jr., a young activist.
Mr. Franks won the votes cast in person at polling locations by 53 percent to 47 percent. But Ms. Hubbard won among absentee ballot voters 78 percent to 22 percent.
Hubbard’s margin of victory was 90 votes. The discrepancy drew the attention of a lawyer for Franks, who challenged the absentee ballots.
The local election board had counted as valid 142 absentee ballots that had not been submitted in sealed envelopes. Sealed envelopes are required under Missouri election law for absentee ballots to be counted as valid. A state judge ruled the board was not authorized to waive the sealed envelope requirement. The judge ordered a new election.
In the resulting special election, Franks won in a landslide with a 1,500-vote margin, garnering 86 percent of votes to 14 percent for Hubbard. The incumbent still won the majority of absentee ballots – 56 percent. But there were only 164 absentee ballots cast."
metmike: Crystal clearly there are many problems with absentee/mail in voting. Those that say otherwise are either in denial, are politically motivated or don't know the facts. In a very close election, there is no doubt that the losing side could be able to use this as a legit reason to question the results.