There's a big advantage to being the default. People are willing to give it a try, and it's reached a point where it's accurate enough now that the rest boils down to personal preference.

I don't have an iPhone, but the one thing I've noticed with AM is that it does a much better job dictating directions ("use the second from the right lane to take the third right onto Example Street" vs "Turn right on Example Street")

I almost never navigate with sound because Android Auto is so easy to glace at, but my less tech inclined relatives / those who don't have Car Play swear by the clearer directions.