I disagree that it's a risky move at all. I have been saying that the first person that can establish a kind of validation of ads will dominate the space. It's only … unfortunately … expected that Google would finally come around to this. They have nothing to loose and everything to win.
Just like you said, they are going to block all the "bad" ads, which will also allow them to control messaging and information access even. We've already seen Google ad network being used as a political tool to undermine and attack ideological opponents. It will really only further enslave the whole internet, communications, and democracy as a whole to Google and further allow them to create a kind of exo-state corporate government that controls what you see and suppress what they don't want you to see for a whole host of reasons, which will even more increasingly become political than they have been.
We have seen it just this last election where Google inserted itself into politics and has been actively working to, undermine ideological opponents by cutting their ad revenue, and is actively involved in propagandizing people where it can through sanctimonious altruism masking the underlying reality that they are censoring on political grounds that side with those they share interests with.
It's quite ironic what is happening these days as the machinations are quite a bit the same as they were in the 1920s-1930s Europe, which led to all sorts of problems including the rise of the very same kind of authoritarianism that today equally claims that hey are acting in the best interest of the people and are sanctimoniously self-righteous. Google does not evil, right? See, we're all fine, because they have our best interest in mind, because they told us so and when I googled snopes for an answer, snopes clearly told me that there was nothing to the claim otherwise.