I have my 3 HP Oneida cyclone in a shed 4' away from my shop wall. It then vents directly outside toward the wall, and the vent has a maybe 45 degree fitting pointing down to the gravel walk in between. It has worked fine for a couple years...almost no dust on the wall. Works great.
Until I get lazy. I knew it was getting pretty close to time to empty the drum, but I wanted to finish sanding some raised panels to get it done while I was in the mood. Next morning I went out while the DC was running, and found that tiny shavings and a lot of dust were floating around the gravel walkway. I waited too long. To make matters worse, the angle fitting had fallen off after only two years because I was too lazy to put a pop rivet in to hold it on.
The roof of my shop was covered in a nice thin coating of sawdust because the outlet vent didn't direct it down. Then it started to rain lightly. Not enough to wash the roof off, just enough to stick the sawdust in place. A friend who was there helped me dump the drum, then I spent about half an hour with a hose, washing my roof.
Nothing at all wrong with the system, it works great.
Let's chalk this one up to operator error.....OK, OK....just plain laziness.