Waze is still in its infant stages. Thats why you encounter some of these problems. What the app does is learn the best routes from place to place over time. The more people that use the app, the more a route is driven, the more people reporting incidents, the better it is.
Read the Wiki, and watch the tutorial video and it will show you what it is all about. You will notice in time the directions get better and better, and also, in time, the routing will be fantastic when it comes to traffic data and construction.