Here is a good example of how a gearbox flush can damage your gearbox.
https://www.youtube.com/watch?v=EwOLHxawq4Q
They did the Volvo flush procedure correctly but their gearbox stopped working very soon after.
It is also worth noting that most auto boxes have a filter than can be changed easily.
Unfortunately, the gearbox needs to be dismantled to replace the filter (stupid design) with these Volvo/Asin boxes.
Which is another reason not to flush them.