
When I was doing this work, I'd say that at least half of them were non-native speakers, along with a lot of students from online diploma mills that evidently didn't care who wrote their papers as long as they turned in something.
That was usually easy money, because these people just wanted something slapped together and didn't care what it was. Nor did most of them have the writing and grammar skills to evaluate anything.
The really irritating customers, who I learned to avoid, were the demanding graduate-level students who were all hysterical about not being able to finish their work on time. Over time, I could just tell by their instructions if they were going to be trouble, so I generally avoided orders about which I had a bad feeling.
Of course, the really bad ones were the ringers, the fake "customers" working for rival research companies who were just out to do as much damage as possible.
Those are the ones that are tricky to avoid.