Opal's a great idea, but it's fifteen years late, and poorly implemented.
Some cities have point-to-point ticketing, some have zones, and others have time-based tickets. Sydney has them all, and the software is supposed to somehow work around that. Opal is the second attempt, as the proposed T-Card was aborted some years ago, as the software vendor eventually threw up its hand in despair at the complex ticketing system. Many cities can send an individual mad, but only Sydney can do it to a corporation.
The Opal software is fine, in and of itself. It's just that the implementation of it was a perfect time for them to develop a new, simpler ticketing system from scratch, but certain invested parties stopped that from happening.
Welcome to New South Wales - incompetent and on the take since 1788.