If they can do the job, then there's no problem at all. In fact, I've been very grateful for my non-native colleagues. Sometimes they can explain a concept faster and more clearly, thanks to their study and training. Admittedly, I had to re-learn a lot of rules before I started teaching. I could follow the rules intuitively, but I couldn't explain them well.
I've heard students complaint that their last teacher wasn't a native speaker, as though it were a form of leprosy. I've also seen students blindly accept whatever an inexperienced "teacher" said just because the teacher was a native speaker... even if the teacher was explaining something poorly. Of course, as students we can't truly know until our skills have developed, so we go by other cues such as nationality.
None of my non-native colleagues have ever pretended to be English geniuses, in spite of their advanced level. I've only ever seen problems online, where a non-native with intermediate English skills proclaims himself to be an expert in English, and misleads learners into thinking that the English he speaks is "perfect". Sometimes he simply lies about his nationality and claims to be native. I see this type turn up on italki every few months.