JazzG As someone who was against Brexit at the time I wouldn't class it as a failure yet. Too early to say anything yet imo. These things need to be judged over a much longer period, it is still very early days.
https://en.wikipedia.org/wiki/Economic_effects_of_Brexit
Everyone who claims to be serious about any of the related questions seems to agree it's been a fuckup economically.
Jury is arguably out on whether it's resulted in a long term reduced dependence on nett migration, but it hasn't reduced nett migration to date, which was the implicit or explicit promise of the Leave campaign.
So that'd make it a failure on all fronts thus far.
I am no great fan of the EU though … ever since the eurozone debt crisis it's failed to make the fiscal reforms that would give it credibility as an economic federation.
The EU member nations who get structural economic benefits from its existence (most notably Germany) would have had to agree to a system of direct fiscal transfers (probably via a reformed ECB) to subsidise the others.
They have refused this, which is one reason EU politics have become so perennially fucked up. Makes Maastricht look like an elaborate risk and debt long con in retrospect.
I agree too that the EU is dangerously undemocratic. The way in which Greek and Italian sovereignty was crushed by the Eurogroup during the debt crisis was far more worrying than most of the stuff people complain about though.