Like anything else in the world, it's not perfect. I used "Find my friends" a while back to locate my wife. It reported a completely illogical (and incorrect location) approximately 75 miles from where I thought that she was. I then used "Find my iPhone" (we have family sharing) and her correct location was identified. I was able to go back and forth and repeat the conditions a half-dozen or so times. Never did figure out why that happened.
She was on the road at the time using the Maps app and she was accessible by the gps birds.