It's based on the lack of scientific evidence for the claims made by NLPers, and the fact that most serious psychologists consider NLP discredited.

I think that a lot of what serious psychologists these days call mimikry is basically what Bandler and Grindler described as rapport building through pacing and leading. Bandler wrote 30 years before Chartrand et al wrote "The chameleon effect: The perception–behavior link and social interaction."

Being 30 years ahead of the time for a pretty simple effect isn't bad.

There no evidence that the original NLP Fast Phobia cure is much better than existing CBT techniques but there is evidence that it has an effect. I also wouldn't use the NLP Fast Phobia cure these days in the original version but in an improved version.

Certain claims made about eye accessing cues don't seem to be true in the form they were made in the past. You can sometimes still find them in online articles written by people who read but and reiterate wisdom but they aren't really taught that way anymore by good NLP trainers. Memorizing the eye accessing charts instead of calibrating yourself to the person in front of yourself isn't what NLP is about these days.

A lot of what happens in NLP is also not in a form that can be easily tested in scientific experiments. Getting something to work is much easier than having scientific proof that it works. CFARs training is also largely unproven.