The EU expanding to the Eastern countries was an extremely beneficial thing for Western countries to do. Since Eastern Europe became part of the single market and westerners don't have to pay tariffs for stuff produced there, EEu has become the China of Europe. They have become the cheap, efficient factories for Europe, without which the Eu would be incomparably less competitive globally against Asia and the US. European industry and economy is having difficulties, as it is. Without EEu, it would be dead.

But is it causing immobilism in turn, like you stated? No. The real cause of immobilism is the archaic laws and systems of the EU itself. It is impossible to control/gouvern such a large and diverse society, with hundreds of millions of people and dozens of countries, while needing UNANIMUOUS decisionmaking. By giving every country veto power, the EU is begging for itself to be immobilised on any major issue. Yes, a few EEuropean leaders are baught and paid for by Putin or just straight up fascists (the previous Polish gouvernment, now Fico in SK, Orban in Hungary), and they have the power to block EU actions. But among this many people, nations and countries, there are always going to be renitents going against the tide. The point is: the EU needs to reform its structures and decision making processes if it wants to keep functioning and being able to hold its own against the competition and straight up attacks (economical, military or otherwise) from Asia or even the US.