[1] https://www.5280.com/2020/05/no-fraud-isnt-rampant-in-colora...
There are states where it is the primary method, and they are (other than overseas absentee ballots, IIRC) in most places counted the same as any other ballots (provisional ballots aren't always counted, but that's a different kettle of fish.) In some cases the process results in them being counted later, which may be after immediate media attention to the counts moves away, but...
Further, mail-in ballots don't tend to lean toward one party or the other at present, so claiming that it's an attempt at Democrat voter suppression is disingenuous. 538 recently published an article on the matter. I would link it, but it's a pain on mobile and I've given you enough info to find the article yourself. This really is about security, an area in which mail-in ballots are lacking.
No, claiming it would be effective Democratic voter suppression might be inaccurate (though the issue isn't “who votes more by mail”, but “who votes by mail that would not vote if only allowed to vote in-person”, which are not equivalent questions, so the fact you relate doesn't even make that weaker case.)
But that the motivation is partisan (whatever the evidence is about the likely effects) comes straight from the horse's mouth: https://www.politico.com/news/2020/04/08/trump-voting-by-mai...
It's not disingenuous to suggest an actor has a nefarious motive when the actor explicitly announces that motive, even if there might be evidence that the act taken would not realize the motivating goal.
> The U.S. state of Oregon established vote-by-mail as the standard mechanism for voting with Ballot Measure 60, a citizen's initiative, in 1998. The measure made Oregon the first state in the United States to conduct its elections exclusively by mail.