The EU's Russian oil ban is a big achievement, but the limits of Western unity will face further tests down the road

London (CNN Business)- The European Union's historic agreement to ban the vast majority of imports of Russian oil by the end of the year is without question a major achievement.

Getting 27 countries, many of which have been utterly reliant on Russian energy, to agree on a package that will almost certainly damage their own economies on the behalf of Ukraine, a country that isn't even in the EU, was unthinkable even a few months ago.

The deal, however, does have flaws that reveal the limitations of European unity and nod to headaches for the bloc further down the road.

First and foremost, the deal does not include oil that is imported via the Soviet-era Druzhba pipeline to Hungary, Slovakia and the Czech Republic.

Hungarian Prime Minister Viktor Orban, Vladimir Putin's key ally in the EU, started Monday by criticizing the EU Commission and calling it "irresponsible" for putting the economies of these countries at risk. He ended the day with a video message claiming: "We have managed to defeat the Commission's proposal to ban the use of oil from Russia in Hungary."

Needless to say, one EU leader celebrating the defeat of a key EU institution is a fly in the ointment for those claiming the deal was a triumph for European unity.

Commission President Ursula von der Leyen said that the pipeline issue would be discussed again, but she didn't predict a timeframe.