Yes, I have certainly seen Google maps do stupid things - sometimes extremely stupid things - when routing. Most of the time it does a very good job.
While you can't optimize by distance vs time, you can optimize for fuel efficiency, which might slightly alter the route though not necessarily in the way you want. But it could send you on some lower speed city street segments than freeware in theory. It won't save time for sure.
The only way to do what you want is set waypoints, I believe, but that defeats the purpose of the routing.
As far as accidents, they can happen at any time. Sometimes, I am in the middle of a drive. Suddenly, Maps becomes aware of an accident 2 miles ahead, and reroutes me. Since I'm not a precog, I turn on the maps for this type of thing. On a recent road trip, it rerouted me to a city street adjacent and parallel to the freeway. I got to see exactly how about that accident was, with everyone else stuck in traffic. Probably saved at least 10 minutes.