It isn't a myth but affects the same wheel and tyre and tube combination in different ways on different vehicles, must be down to variables like driving style I guess.
Cheap tubes are often not too good, Michelins are good, and removing ALL labels from the inside of the tyre is a must.
I have to say I wouldn't do it because it loses one of the main benefits of tubeless which is slow deflation if you get a nail in the tyre at speed on the road, but that is just my 2p. I have never had problems with losing pressure due to mud in the bead of tubeless tyres though, some say it is a problem on BFGs but not for me.