As Myra Mains said, they're good for what they're designed for - creating a layer of mist over a water filled feature.
I don't believe the small ones are intended for any kind of outdoor use, and even the larger ones meant for fish ponds and the like won't make a fog that's any good for filling a graveyard.
The mist they make sits well over the water they're created in, and it can spill over gaps in any enclosure (like a space between stones around a pond, or if a bowl the fogger's in has an uneven rim).
I like to use these indoors, where there's little to no breeze - but even the breeze of someone walking by quickly can disrupt the mist for half a minute.
I have used these in a skull (which I modified to hold water, but allow the mist to pour out the mouth) and in mini cauldrons, both of which would allow the mist to pour out, and into a larger cauldron with chilled drinks inside. The cold air will help keep the mist around longer, and it's enjoyable for guests to reach into the "mysterious mist" to grab a drink.