I think that, when a method of voting, such as mail-in ballots, is new and untried, there is some reasonable legitimacy in doubting the practice and wondering how secure it is. It is reasonable to make sure that it is evaluated carefully and run fairly.
When it hasn't been done yet, you can say "prove it works." Once it has been done and has worked, it's not so easy to say that.
Here in Vermont the governor and others in his party were quite dubious that mail-in balloting would work well. They worried not only about fraud but confusion and bureaucratic bungling. But the state is politically divided, and the Democratic administrators in charge of elections carried the day. So, instead of whining about it and making a big fuss and lying, the governor decided to do his best to make the plan he did not like work better than he feared it might. Not totally surprisingly, it ended up working.
Granted, Vermont is a small and traditionally bipartisan state, and the leaders of both parties are not a pack of thieves and liars, but I think it's possible to run a good election, by mail or otherwise, if the people in charge do their job properly.