Are you talking about absentee ballots for people out of the area or handicapped or military, or the wholesale mail-in ballots for anyone who wants one?
In everything you balance the pros and cons. Absentee mail-in ballots were necessary for some people to keep them from being disenfranchised. Perhaps there was some fraud, but I' pretty sure it wasn't enough to change anything.
Now, multiply that number of mail-in ballots by who-knows how many times. Even if the percentage of fraud stayed the same it is now many times more than it was. And you can be sure (at least I am), abuse of mass distributed mail-in ballots occurs at a much higher rate.
It it a perfect vehicle for organized fraud.