Arguably Brexit has already failed and it had nothing to do with the EU.
Despite the rhetoric no one seriously expects that the UK is going to have an economic “Brexit Dividend”. That’s not the EU’s fault, it was the PM that decided the UK’s red lines to exit from the single market etc. and that has inevitable economic costs.
As for setting up the UK as an independent free trading nation, the the UK government has balked at the prospect of going it alone and looks like it it intends to cleave to the EU and its regulatory level.
While in terms of reducing immigration, there are signs of immigration substitution from the EU to ROW but still at the same numbers. Though that’s still in flux so who knows.
Taken as a whole, we’re firmly in BINO territory where the UK has swapped a formal relationship with the EU member states, mediated through the treaties, for an informal relationship with very similar results, except the UK will not be at the decision table.
As for the EU’s approach to Brexit you may or may not like their 3 priorities of the UK’s financial commitments, EU citizen rights, and the border, and the tacit position that single market membership means total acceptance of single market rules and ECJ oversight, but the EU has conducted itself transparently throughout the negotiations. There were no unexpected demands from the EU and if the UK was caught on the hop with the backstop it’s hard to see how that can be laid at the EU’s door.
In the Withdrawal Agreement the EU made a huge concession in allowing the UK to have the extendable EU membership-lite to 2021 and another in the total UK customs union so long as it is necessary. Further in the talks Barnier’s team made clear to the UK that even out of the single market, the intention was to treat the UK’s products extremely favorably with exceptionally low levels of regulatory checks.
The UK can point to hardball over totemic issues like France pushing fishing rights or Spain on Gibraltar but the EU could have been significantly more awkward in negotiations.
The EU was never going to cheer lead Brexit however if you had said to Brexit campaigners in 2015 that the EU’s 3 demands were finances, citizens and the border and that the EU would publish those demands openly, I suspect they would have anticipated the UK would be in a really strong position.
That they’re not says more about Westminster than Brussels.