I purchased one of these back in August of 2016 based on good reviews on the 2x version. Auto focus works fine with my gear (1DM4, 70-200 IS II USM) & pictures are sharp. However, I've discovered that it drains the battery on my 1DM4 when the camera is on or off. I at first assumed that my battery was nearing end of life but just recently realized that the battery drain only happens when the extender is installed.
Warranty? While technically, it's still under warranty, the warranty appears to be virtually worthless. Yongnuo contacted me and said that I must pay shipping to China both ways, they won't provide an estimate for length of time to repair, and they still haven't provided the details I need to return the extender. Simple things like a shipping address and what documentation to include has not been communicated. I've asked for this basic information and we'll see if I ever get a response.
BTW, any idea what shipping to Hong Kong will entail? I'll probably just cut my losses and consider it a lesson learned, but I'll keep you posted on developments.