The last example of a synthetic nation made up of wildly different elements and dominated by one group was Yugoslavia, and we know what happened when that went bang. And there is the rub, the European Union, in its drive to form a United States of Europe, will slowly pull itself apart, and when it does, history suggests it won't be pretty.
The EU is currently drinking its own Kool Aid, but at some point, will have to face up to the fact that it's model of repeated compromises across 27 members doesn't equate to a coherent command and control in a time of crisis.
However, whilst this process follows its predictable route to failure, those nations spending >2% on defence are expected to subsidise the defence of the others, including the EU's wealthiest member, Germany, which has allowed its defence capability to descend into a shocking farce.
Another example is Belgium, which having been invaded twice in living memory, spends half the recommended NATO level on defence, whilst relying on the contribution of forces from The US, UK and others in a time of need.
The 'Donald' is spot on, and the Europeans, with their contemporary outrage at his directness, have read this one very badly. It's not Trump putting NATO at risk, its European intransigence.
Of course, one example of a "synthetic nation made up of wildly different elements and dominated by one group" that you omit to mention is the UK. We may yet get to see what happens when that "goes bang".