If you want to say this (which I guess you do, so your contact understands) without sounding like a bigot, focus on the country, not the client.
"I've found X country difficult to work in."
"I've found the business practices of X country difficult."
This is a valid issue in some industries. You just need to frame it right. The below is from my own experience. Does it sound bad?
We had difficulty breaking into the market in the petroleum industry in Latin America. Many oil companies there are state owned, which means they are inordinately slow and clumsy at making decisions regarding new technologies (not good if you've invested lots of time and money demonstrating your product.) In North America and Europe most large oil companies are privately owned and we were able to sell much more easily there.
