One thing I noticed shortly after when I initially switch Arc a couple years ago from Firefox, ads were more targeted at me and I definitely felt more tracked online. I started using Zen Browser at work because they blocked Arc for Windows (Zen blocked as well but I can run the Twilight build without being blocked :D) and the more I used it I just like it better than Arc in general and it was nice to be back to using Firefox again. When Arc went into maintenance mode it was just that last thing to finally get me to move off of it and over to Zen everywhere.
Arc is based on chromium I believe and that has manifest v3 which means poorer ad blocking afaiu. I won't touch any chromium implementation with a 10 foot pole because of this exact reason. Arc also does not allow you to export bookmarks natively. i had to use a 3p script to do it
Honestly I think Dia and any other AI first browsers are a bad idea.You lose out on the exploratory nature of the web. How are you going to stumble upon a random nugget during a web search session that you never would have found otherwise. This kind of thing happens to be frequently when I don't go to AI to figure something out. I find a new product or new library for the language I am currently working with that may not solve my current issue but I come back to later to improve something I am working on.
The web as a whole is also built on this discovery and people visiting pages and interacting with other. This gets lost when you get into the flow of just AI'ing everything.