Hot flush is best for cleaning debris out of trans coolers during trans replacement. I don't think they make a machine for fluid replacement in a working trans.
The big kicker here, is that people wait until the trans is failing to change the fluid. So you have a worn trans with bad fluid, starts acting goofy. Triggers a light bulb in customers head "better change the fluid". Trans is already fubar before they bring it to you. New fluid won't fix it. Trans goes out shortly thereafter. Guess what caused it (in their mind)! Must've been that damn trans flush they did!
THAT scenario really does play out in real life across the nation. And that is where those "horror" stories come from, and why people will swear on a whole stack of bibles that those machines will cause a failure. It won't hurt a good trans. And it won't fix a bad one! It's like getting an oil change to fix a rod knock. Tell the customers right up front, "It's preventative maintenance, if the trans has been acting up on you, this won't fix it." We used to.
