Those are extreme examples but it is not a stretch to say there are people who go to a country and have no interest whatsoever in contributing to that country. Look at all the British who form little Britains in Spain for example. I doubt they could claim to have been forced into it by prejudice. In reality it's more about sticking with what you are familiar with. The issue is that can carry through the generations if people don't mix outside that enclave. Most people do mix and by 2nd and 3rd generation they consider both the enclave and the rest of the country as their home, but for some they live in a country they don't really consider home, other than a smaller area that is more like what they left. It doesn't have to be malicious, it's just human nature. It's definitely a thing. The point is any example that as given could be dismissed. Also you are missing the premise entirely, it's not that we should be wary of people from x or y or who look like z at all, it's about what the individuals can contribute regardless of where they are from.

Also things like religious extremism or people with strong, conflicting loyalties to the nation they left are not unheard of. The argument that there is no immigrant who isn't a good immigrant is quite clearly just as false as all immigrants are bad immigrants.