Mass surveillance provides unparalleled power to those who control it. It's unprecedented in all of human history. I don't believe humanity knows how to safely handle such power at this point.
One of the most fundamental concerns is the fact that, once collected, the data is vulnerable to being misappropriated or weaponized for the entirety of its existence, possibly for many decades or longer. By that point, laws may have changed significantly and the entire political climate could be drastically different.
Imagine being automatically put on a special list and thus denied certain basic priveleges due to "questionable" posts you made in an online forum 15 years ago. We can imagine a wide variety of vague and controversial definitions of "questionable content" for this thought experiment.