First, even for economies which are capital scarce, the developmental value of short term portfolio capital flows appears tenuous. Second, exchange rate volatility triggered by inward or outward capital surges can be very disruptive to the real economy and the banking sector, especially if domestic agents have foreign currency liabilities.
The main drawback of capital controls is that they are often not very effective, because private agents find ways to circumvent them if the incentives are strong enough.