I have found it is sometimes a bit off in the route it will navigate, and I do not understand the rationale. On my phone I of course have Google Maps and driving to a destination I know well (i.e. best route to take) and while Google seems to be better at giving be the best option, the Y's route is usually the most direct mapwise but certainly not the quickest or most efficient.
Where I live I have the option of taking multiple country roads that lead into the city. Coming home I sometimes take the back roads because there is less traffic, I can drive faster, and is actually more direct. When I turn off Google usually re-routes to the path I know is the best, Y annoyingly keeps trying to direct me back to the main highway (that is the worst option). One night I relied on the car's navigation I realized later it had really chosen the worst route possible. It had taken me on a longer path so it would get me back to the main highway it seems to prefer. Problem is if instead of heading directly south I had gone south-west I could have gotten on the same highway 10-12 miles further towards home.