Very few maps, no simulation or indications for where congestion or overflow is happening which would have made the game more interesting and, with any amount of game state feedback, easier for the player to determine what works and what doesn't (whether we're succeeding or failing).  Instead, fixing the final bits of a nearly winning design feels like guessing rather than deduction, and it's easy enough to do (no penalty for infinite guessing, nor reward for getting it right the first time or in a minimal number of tries) that it becomes guessing.