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.