The problem with unofficial euro countries is the same as with the official ones. They have no control over monetary policy. If inflation happens they can’t do anything about it.
In such a situation, they are like the states in the USA. They have a “treasury” or “central bank”, but it basically has bank accounts with the rest of the world. You need money, you borrow. You need to pay bills, you write cheques. If at some point the rest of the world won’t cover what you promise to pay, you are broke just like some unemployed auto worker.
At least for the unofficial countries, it is simpler to create their own currency. They can do it any time, pass a law declaring that for all transactions they have jurisdiction over, the new currency (Montenegran Euro-Dinars, say) is now the currency. Then, they can control the fluctuations.
The difference is why they have to do so. Greece has promised so much money to bond holders and pensioners and civil servants, that soon the banks that pay those bills will want payment; if Greece has no money, it has to borrow to cover that; if nobody will lend them, they default on the loans and the pensioners and civil servants don’t get paid.
Normally Greece would print more money, it’s central bank would pretend the money was there, and everyone would be paid. They only problem is, outsiders who see this behaviour would want more to cover future inflation, so the Greek currency would become worth less (2 words) and so in relative terms, everyone takes a pay cut from the US dollar point of view.
The trouble is the moment there’s a hint that Greece is quitting the Euro, everyone will know this inflation/devaluation is coming, and they will all rush the banks to get physical euros before the government declares their accounts to be in Greek currency; the banks would go broke due to these runs on the bank. All the debt owed to foreign banks (a lot of it) would still be in Euros, so devaluation won’t help.Or else they don’t, the euro takes a dump an is devalued for all of Europe.
If Montenegro is basically solvent, then it can ditch the bad currency anytime for their own, and people would probably rather have the Dinars than Euros. Nobody expects the value of the Dinar to fall, so trading proceeds smoothly. Much smoother transition. Banks might be happy to sign a new deal, to accept M’s debt in dinars instead since they won’t be devaluing like the euro might.