This statement may be your personal experience, but it is total rubbish provided the 6x9 folder in question has been properly serviced.
While certainly not the equal of a large format camera or a modern 6x9 with the lens mounted to the body, even a second tier 6x9 folder can produce crisp enlargements from an f/16 exposure that will simply blow the box camera out of the water and its exposure control is so much better.
That said, there are some hurdles to overcome when shooting with a 6x9 folder. The big one is that most of their current owners will not invest either the money or the time to completelly refurbish the camera. The main difficulties here is making sure that the range finder is accurate, the shutter is functioning properly, all the elements are cleaned, the front element is collimated properly at infinity and the lens assembly is parallel to the film plane. Secondly, one needs to wind the film just before making the exposure. Third, these cameras beg to be used with a tripod, as they are rather difficult to hold steady other wise. Also, a lens hood is a must have accessory.