I travelled back to US from Abu Dhabi pre clearance in 2017 and it went pretty smooth. There are only couple of flight services are eligible to clear US customs at AUH, primarily Etihad flights. I had to wait only 5 minutes in secondary inspection to get my AP back and no questions were asked. This year, DFW was the port of entry and also went very well but had to wait around 20 mins in secondary but again no question was asked even by primary officer as he just took my bio metrics and sent to the other room for additional paperwork....