Reddit reviews All Out War: The Full Story of How Brexit Sank Britain's Political Class

All of this stuff is really overblown. It's nice, fits into a narrative of shady billionaires psycographically manipulating people and entering their minds etc etc but there isn't actually much to it beyond pseudo conspiracy theory narratives.

Clinton's campaign, for a start, had access to a lot more information due to having the databases that Obama's team had built over the previous elections (except for a lot of data that the Obama team had used in a third party app that allowed them to target specific people, which Facebook banned because they considered it's use unethical.)The Obama team are the original masters of this sort of stuff, and it's likely that if there were an information/data disparity, it would have favoured the Clinton campaign. There were some shady things Trumps team did regarding 'dark posts', attempts at voter suppression, but there's no evidence they had any effect. Social media is a great way to reach people, but it is not yet really a great way to read them.

Regarding Brexit, it's even further off. Dominic Cummings, the head of the leave campaign and an expert on data use in campaigns, has also said that Anything you read about Cambridge Analytica, 'psychographics', & social media in politics is ~99% likely to be bullshit because the Big5 personality adds relatively little predictive pwr. Demographics overwhelmingly more important for predicting &/or digital comms.
If you read Cummings' 20,000 word post on the referendum, in which he outlines the techniques thy used or read Shipmans book on Brexit All Out War you'll see that the Brexit campaign focused mostly on demographics, with Vote Leave geo-targeting people between the ages of 35-55, who lived outside of Scotland, London and areas with high UKIP vote. They did this with a massive social media post blitz in the last 5 days of the campaign, a strategy they called 'Waterloo'. (E: note this was not the sole reason they won, when votes are as close as they are in Brexit or in the Trump election, it isn't really possible to reach 'one single reason' as to why something happened, despite our natural human bias to seek simple narratives that help us understand complex things.)

Again, there's a nice narrative here, and Cambridge Analytica do all they can to play into it, with their University town/fake latin word name, and a shady posh english bloke doing their presentations, they have crafted themselves a niche as the evil geniuses out of an 80's action movie. But there is very little cold hard evidence in the article that speaks to the effectiveness of the 'big 5 traits' methods. It's not even new, its been in use in various fields for decades.

Recommended reading: All Out War

Hijacking the top answer to add that All Out War by Tom Shipman is an excellent read if you want a deep dive into what was going on behind the scenes in the months leading up to the vote. Much has happened since its publication, and if I recall correctly, there's a revised edition out, containing updates from 2017-2018.