Essentially, the problem here is the inversion of politics from the advocacy of a set of policies, and convincing people to join you on those policies, to an election where the number of campaigns is equal to the number of voters and you can use the micro-targeting to take these campaigns to the individual level.
In the United States, it was used to suppress the vote. I can't speak to exactly how it was done in Brexit, but it's very obvious there was a dramatic effect there.
The essential point here is whether you believe that one can have a healthy democracy in an environment where there is advertising that's completely unaccountable because the only people who see it are the intended recipients.