Everyone talks about the danger of leaving the EU while forgetting one very important aspect.
What's the danger of staying in?
In France Marie Le Pen is fast gaining a lot more support and is considered the biggest treat to Emmanuel Macron ( https://www.bloomberg.com/news/articles/2019-01-13/france-s-le-pen-starts-eu-campaign-promising-win-for-patriots ). If she wins the next election France is highly likely to leave the EU, and if France leaves it's game over for the EU, it's finished.
You can also point to Italy. What they are doing is seen by many as even more damaging than Brexit to the EU ( https://www.bloomberg.com/news/articles/2018-11-22/the-eu-is-more-worried-about-italy-than-brexit ). If Italy keep making their own rules, why should other EU states follow the rules?
Well it turns out other country's like Poland already are ignoring certain rules ( https://www.reuters.com/article/us-poland-nationalism-special-report/special-report-why-poland-fell-out-of-step-with-europe-idUSKCN1MS1M3 ).
You could also point to the huge Economic problems of Greece, Spain, Ireland and others to see where the next wave of populism is likely to crop up.
If you ask me, the EU is not in good shape at all and collapse is looking more likely every year.
The UK be wise to leave the EU, take the painful couple of year hit while getting it affairs in order, because by that time the EU very well could collapse and the UK will be left as the most powerful and stable county in Europe.