With ads there is incentive for tech companies to harvest more and more data points for advertisers to use their magic orbs to find out whether you like Diet Coke or Diet Pepsi more. This in turn leads to powerful and concerning levels of understanding of a certain individuals personality and worse, behavior, that allows them to exploit those flaws in your personality, or exploit your emotion more than a billboard ever could.

On top of this, it creates an unregulated sky-is-the-limit 3rd party economy of data brokers who will sell these bulk collections of data to whoever coughs up the money for it. Enter in governmental intelligence agencies, there now is a concern of governments either sidestepping laws on lawful intelligence collection or outright breaking them, as the NSA and FBI have already admitted too if I recall correctly.

Edit: I'd also like to say, can you imagine how the intelligence agencies would pressure their legislators during attempts of reforming data brokers? "You can't take this away! We stopped this attack from happening with this! If you take this away, you'll cause 9/11 2.0!" That's exactly what they did for the FISA re-authorization this year in the US.

Regardless, personalized ads are evil and should be banned at least for granular types. People will survive with less personalized ads but we'll avoid a lot of problems by ditching them