You first statement is not correct. The rule is perfectly useful to solve mazes where the goal is in the center.
The requirement is that the walls should be connected, i.e. that there are no free-standing walls somewhere in the maze not connected to the border walls, for example.
Yes, in a maze without cycles the left-hand rule will take you through every part of the maze, guaranteeing that you reach the goal.
Of course in a maze with cycles you might still reach the goal, but it's not guaranteed. For example there's a cycle in Hampton Court Maze, but the left- and right-hand rules both get you to the goal from the entrance.
They are referred to as "multiply connected" in the literature on mazes. Essentially, this is the mathematical way of saying that there are "islands" of hedge, not attached to the perimeter.