I mean a camera from a "questionable" source, IE one of unknown reputation. While I haven't bought any Dahua cameras from Amazon, Alibaba or Flea Bay, the cameras I have bought from those sources have all been disappointing in one way or the other. The Dahua cameras I have, currently six, have all come from Empire Technology, Andy, and have met every spec with no problem.
The big problem being that Dahua will sell cameras that have been "stripped" of some features, both in firmware and hardware at a lower cost. Vendors buy them and re-sell them as if they are actually full, OEM, cameras. When an update is applied it can't run because too much of the camera is missing. Unless you're sure of the supplier, be very cautious about upgrading firmware, especially if it's working fine.