Mail in voting is subject to abuse. What control do they have? The most common form of abuse is paying someone to sign a ballot and then filling it out.

How do these states you referred to detect fraud in these instances?

That is why most countries limit mail in voting to those that request a ballot and have a hardship.

The fact that some states in the US do not does not make it safe.

Keep in mind a US Congressional seat was won by 4 votes certified by the state and Pelosi initially refuse to seat the person. In 2000 Democrats raised issues about mail in votes in the Gore election leading to a commission chaired by Jimmy Carter which concluded mail in voting was subject to abuse and should be restricted.

Hillary claimed Trump stole the election in 2016 with the help of Russia.

How can you claim the controls are adequate when challenged as being inadequate when Dems lose close elections?

The reality remains anything less than confirming the identity of the voter at the polls is subject to abuse. Usually the elections are not close enough for a little abuse to influence the outcome, but that is different than saying it does not happen.