FILD has worked for me a few times (maybe 10-20% of total attempts). The failures have always been from falling asleep and losing awareness and concentration on the finger movement. But, no harm done, at worst I've missed a couple of minutes of sleep in the process. I'm also relatively new to this, I expect it could become much more reliable with more expertise. I like this technique because it's easy to do, doesn't disrupt your sleep (I always wake in the middle of the night naturally), and doesn't detract from other LD attempts later in sleep, including DILDs.