We are in the beginning stages of adoption, so I'm not an expert, but I can pass on some of what I've learned in my research. An adoption doc explained to me that the risk of attachment issues varies by region (how nurturing home culture is), age, length of time spent in an institution and pre-institution life. A child who has attached to caregivers or others early in life will have a lower risk of attachment issues. Also, the less time an orphan has spent in an institution, the lower the risk. So, a child from a loving background - no matter how difficult - who has spent a limited amount of time in an orphanage in a nurturing country may have a fairly low risk of severe attachment issues.
Also, there is a range of attachment issues - the severity of which probably depends somewhat on your own definition of "severe." You might look through the medical archives both on this site I'm sure there are other experienced aparents that can direct you to more resources.
One thing I'd suggest is to contact a large reputable agency with programs in a variety of countries and talk through your concerns with them. They would probably be able to delineate which areas/types of akids are higher risk. Perhaps you will find something you are comfortable with. In the end, there are no "guarantees" in adoption but don't give up without informing yourself thoroughly. There are way more "success" stories than tragedies!
Blessings,
Christine