The advantage of the EU is that it makes trade among it’s countries easier and reduces the threat of war among them also. The problem is that there are many member countries that are basically bankrupt such as Greece Portugal Italy and Spain, who are a drag on the economy. Basically if you are a richer country you will have to bail out a poorer country. Also there are no more border controls so criminal can migrate from country to country to avoid prosecution. Similarly, workers can move from country to country driving down wages.
As far as the EU eliminating wars in Europe, I think NATO played a bigger part in insuring that there isn’t another war in Europe than the EU. It is basically the equivalent of the UN for western Europe and is just a paper tiger and doesn’t really accomplish much. This is unlike the United states which is one country made up of 50 states who are united by a common language.