Ireland, Portugal and Spain economies all tanked when the financial crisis hit as their economies were unbalanced due over development of housing projects. France suffers ongoing economic turmoil due to it's own cumbersome legislation and Germany risks unbalancing the who Euro region due to it's exports/imports ratio. As an outsider BBB looking in the whole Euro project looks like protection racket to protect German businesses and that's before we get onto Greece and Italy.
French GDP is now bigger than the UK's, thanks to Brexit. The cumbersome legislation is their own, it's not imposed by the EU. The German economy is much better developed than ours; widespread high technology manufacturing and R&D, high citizen education levels, high national minimum wage and cheap house prices compared to the UK.
The French health care system is more effective than the NHS. Their state run health funding scheme works better than ours does. Germany's Gesetzliche Krankenversicherung system functions better than the NHS funding system and public health care standards in Germany are considerably higher.
We are not in the Eurozone and never will be. Irish, Portuguese and Spanish house prices fell off a cliff because of new-build oversupply and because their base rate was not slashed by the ECB to the lowest level for 321 years (unlike our base rate was by the BoE), nor did the ECB trigger several rounds of quantitative easing (as the BoE did). Our house prices dropped by at most 25% and then went right back up again. For the last decade the UK has built the lowest number of new homes each year since the 1920s, (great for property owners and the elite but bad for first-time-buyers). Instead of trying to encourage the development of a high-tech manufacturing economy (like Germany), successive UK governments have taken the easy way out by encouraging the service sector, international financial services industry, low paid/insecure employment and property speculation.
It is not the fault of the EU that ~800,000 unskilled Poles moved to the UK between 2004 and 2011. Tony Blair decided not to implement any restrictions on immigration from the countries which joined the EU in 2004. (In contrast, Germany decided not to open up to unskilled workers from Poland until 2011 and France also delayed it for several years.) EU law does require its member states to offer the same benefits to EU citizens as they do to their own (if those people satisfy certain requirements). However, EU law also states that members can force EU citizens to leave if they have no means to support themselves after 6 months.
Greece should never have even joined the Euro. It was proved that the Greek economy had not met all the five accession criteria for joining the Eurozone (as they submitted manipulated budget deficit figures). Italy has a serious problem with government corruption and the embezzlement of public funds, the EU can hardly take the blame for that.