I don't mind ads, I understand that websites need to finance themselves to cover their costs (and maybe build up some capital to expand). But I do mind tracking, user profiling, personalization / user targeting, trading this data with dubious companies worldwide, and obnoxious ads, for example pop-ups or auto-play videos with a 1 micron sized close button, or a forced timed ad which is hiding the content.
It's like having a bunch of people following you around, taking note of everything you do, evaluating that data, making statistics, dicsussing it with other people you don't know, etc.. Then, when you want to make yourself a sandwich, step in between you and your sandwich, taking up a megaphone and scream into your face : "OH, WE NOTICED THAT YOU ARE MAKING A SANDWICH. CAN WE INTERST YOU IN NEW FANCY BUTTER KNIVES FOR ONLY 59,99 €?" [Then going on about it for 3 minutes before they are stepping out of your way].
There are laws against that in real life, and in the digital realm this is missing. Considering how much time a lot of people spend online this is something which needs to be taken seriously.
It's really scary sometimes. There was a time when I was stupid enough to use facebook, just to stay in touch with friends. Once I talked with a friend about allergies and asthma, and I told them I have a pollen allergy. A short time later an ad showed up on my facebook feed, advertising some nasal spray for allergies. Wtf?! And that's just the surface. "Harmless" ads. Who knows what else happens with that data?
And then we get stuff like Cambridge Analytica.
Filter bubbles are one thing, which I find is a huge disadvantage to personalization. You'll never learn about new stuff, because it will never be presented to you, since someone assumes that you blong to a specific box.
Another is that I value my privacy. It's no one's business what I do, when, where, with whom and how. Apart from that, there is no guarantee that this information is not being misused.
For example, I'm thinking about political campaigns, which target specific user groups on the one hand, or spread misinformation and distrust to others. I see such forms of information steering as detrimental to democratic societies. Free and unbiased information is crucial for critical thinking.