What problems does expatriation raise?

From what country to what country?

In general, anyone who immigrates to (or even lives temporarily in) a foreign country has to make a tremendous number of adjustments and is subject to extraordinary stress. Most people really don't make it, in the sense that they can really adapt to another country, customs, language, etc. Very few are sufficiently educated and aware, flexible, adaptable, have the strength of character & personality, etc, to really maintain a sense of self yet adapt to an entirely different culture and surroundings.

As for specifics, an American citizen who expatriates (unlike citizens of many other countries) remains subject to US income taxes on worldwide income. Also, even if an American citizen becomes a citizen of another country and renounces US citizenship, they remain subject to US income taxes and tax laws for 10 years after renouncing citizenship. And now there is new law which mandates that expatriating American citizens and long-term residents (8 years) must pay an exit tax: 42% of all worldwide assets, including unrealized gains. So if your British granny left you a set of heirloom pearls in her will, you pay 42% of the value in exit tax if you move to the UK to join your relatives there, whether you sell the pearls or not, whether you were ever a US citizen or not.