Second-hand dust from a well maintained dirty-fan cleaner is a potential quirk of the design, it has to be said. However, I would still suggest that it was not all that much worse than a clean-fan machine which had a cloth bag and no filtration (of which a good deal cleaners were built in this fashion).
The real problems in the dirty-fan cleaners arise from when the plastic ducting and bellows wears out, or cracks, or splits, either through wear and tear due to the handle constantly being moved up and down, or through solid objects hitting the fan and smashing their way through the cowling around of the fan.
As well as this, a good deal of cleaners had poorly fitted bellows and ducting etc. right from the assembly line at the factory. If a cleaner has closely sealed parts and the bellows are intact and a decent bag is fitted, second-hand dust should be minimal.
However, in the reality, papers bags would often burst, leaving layers and layers of dust and filth in a soft bag cleaner, and spraying it into all corners of a hard bag cleaner, often into spaces which the average consumer would never think of or be able to clean out.
Poor quality dust bags would indeed only add to this. Of all the dirty-fan machines I have ever worked on, the Electrolux 152, 160, and 170 stand out from all the rest as being one of the best and easiest to keep clean. A filter in the front of the hard bag did prevent a lot of dust escaping and the hard bag itself was easy to take off and wash out with soapy water. With the exception of the filter pad, there were no other parts fitted to the inside of it. It was just like cleaning out a small household pedal bin. Where this cleaner lost points was in the tee joint where the main handle connected to the cleaner head, because given enough time, the joint could wear out and cause dust to escape. But I still thought it to be the best design of hard bag. I cannot tell you how much dirt and filth I have removed from behind cordwinder units and electrical panels on the likes of a Hoover Turbopower or a Moulinex.