The fraught with danger is that it is very easy to under or over fill the transmission oil since there's no gauge or dipstick.
If there is insufficient oil in the transmission, the components will wear faster, or worse, fail.
Too much oil and the transmission "drowns" and gets air logged, which affects the flow of the oil and causes overheating.
The oil overflow technique is also not a very reliable way to ascertain the correct amount in the transmission components as there's oil hiding in the oil cooler and hoses.
Also, to get sufficient oil to reach all the transmission components, the engine has to run, gearbox engaged, and the oil has to reach 40 degrees and pressure has to get to 5 bars, all while pumping oil from under the car on the side of the transmission wall while excess burning oil is overflowing out at the same time. I could see the guys working on the car flinching as the oil burned them.
Bmw dealers have ATF machines that automate all this but indy workshops usually do not.
Cheers. Howie