There is some truth to that, but not never. A flush will dislodge any metal shavings and crud from the moving parts. The filter should catch these, but the filters themselves can get clogged, and then bye-bye transmission.
Flushing can really be bad if you've never done a routine flush on a schedule. You don't want to go 150,000 miles before your first one. You would need a garage with a forced flush system to move it all out, and then probably flush again soon after to make sure all the gunk is out.
Transmission oil breaks down with heat and wear like any other, and will eventually contain sludge and dirt.
Flushing can really be bad if you've never done a routine flush on a schedule. You don't want to go 150,000 miles before your first one. You would need a garage with a forced flush system to move it all out, and then probably flush again soon after to make sure all the gunk is out.
Transmission oil breaks down with heat and wear like any other, and will eventually contain sludge and dirt.