As far as the scoring system:
Corner goals: the ramp brings the balls over a light bar (like the ones in your chool cieling), which allows a camera-like sensor above to count them. Since the hole in the wall is only tall enough for one ball, it can easily count any number of balls left to right.
Center goal: the sensors are on either side of the sloped ramp, aimed across to the other sensors (so, aiming parallel to the front side of the goal). The goal’s internal ramp is only deep enough for one ball really, so the sensors dont have a problem there either, since they pass down the chute one at a time.
What I’ve heard as far as reliability “not 100%, but pretty darn reliable”
As far as human interference:
The ball corrals on the corner goals are very long (as far as from the wall outwards) so an HP would have to intentionally try to interefere with the system by leaning way out over the “corral” and extending his arm all the way, so it would be rather obvious if this happened.