This is the significant issue.
Where I've seen fenced, no-trespassing areas, I treated them just like deep water. A disc in there, is a goner. And I love water on courses.
But if you've got a situation where disc golfers will trespass---and if they can, they probably will---the course gains a bad reputation and could end up getting pulled.
This has been my experience as well. Hole #11 at the Disc Creek course here is next to an area that used to be occupied by the Fire Dept. training facility. the tall fence surrounding the area had a few areas that disc golfers pulled up the bottom to go under to retrieve discs that went in. The Fire Dept. guys didn't seem to mind and they even left the gates open, even though the gates were on a different side than the course.
Several years ago, the Fire Dept. moved to a new facility near the airport and the Amarillo Police Bomb Squad moved into the vacant buildings. They put up "NO TRESPASSING" signs all around the outside of the fence and repaired the holes in the fence. Still Dg'ers kept going into their lot until one day a golfer went in at the wrong time. He got arrested (not sure if it was for trespassing or the "unmentionables" he had in his possession). I was able to work out an arrangement with the APD to get discs returned, but the golfers have a difficult time learning that some people don't want us in their area.