I don't think Brexit was awful, and I'm not certain history books will paint it as a disaster. Currently it's seen as awful because of economic impact and because of the desire to have a strong Europe to oppose Russia, along with the insinuation that Russia was trying to engineer the breakup of Europe. However, a United States of Europe would also have been a bad thing. Unions, federations, and so on are tricky because minimal collaboration between equals is ideal and maximal central government is what tends to emerge. I know it seems irrelevant in the current climate, post-covid and with Russia rampaging around, but Brexit had an overlooked role as a backlash against that.

(It's a valid role if you're troubled by giant octopus-like central governments, I mean. Not everybody is, and I know this is an unpopular opinion, as will now be redundantly demonstrated by downvotes. It is however an aspect of the picture.)