The tunnel has an added challenge to it in that the dog loses sight of the handler for some period of time after which the dog has not retained sufficient cues as to which direction to turn to at exit, or if to turn at all or to blast out in extension.
Less of an issue with typical AKC courses, somewhat of an issue with some USDAA courses & more frequently an issue on international courses.
The challenge is actually for the handler to properly cue both the tunnel & give cues as to where to go on exit before entering.
Even many dogs who are tunnel suckers will have such trouble if the handler hasn't worked with the dog sufficiently in training to successfully maneuver courses with non-obvious changes in direction, especially those that require lead changes against the curvature.
With other obstacles the dog still has the handler in sight to assist during the execution of the obstacle if necessary.