It's not about ads. it's about manipulation and gathering data to be used against you at every and any opportunity. The data that's sold and passed around is used to determine things like how much you pay for things vs your neighbor. It determines what services you are offered and at what terms.
If a store thinks I make more money than you, I might for example be told they've got a return policy that is much more generous than what they'll tell you. There are places where you will wait longer on hold, getting repeatedly bumped back to make room for people who have a better "consumer score" than you do.
The data is handed over to the state and locked away until it's useful to them. It's can get you arrested or questioned by police when you've committed no crime. It can get you sued for things which should not be crimes but are. It's used by politicians who carve out the borders of their districts specifically to limit the ability for your vote to make any difference.
That data is used to target you with lies and misinformation. It allows you to be targeted by extremist groups. It can be used to decide if you get hired or not. You could be turned down for housing because of it. There are millions of things that might prejudice another person against you. Your lifestyle, your politics, your religion, the people you associate with, your sexual history, your medical conditions, etc. You can't know who is accessing your information or for what reason, or know how accurate that data is, or know if/how it is being protected.
It's a gross violation of your rights, which yes, sometimes is used to manipulate you into buying something you might not have bought otherwise, but increasingly is used for much much more and once the intimate details of your life have been taken from you, you can never take them back or prevent them from spreading.
However much you enjoy the products that are spying you, you can't ever say if the benefits outweigh the costs. You aren't even allowed to know what the costs are or when you are paying for them, and there will never come a time when you have paid in full and it can no longer cost you something else.