Yep, what he said 100%..
Let's take Rolex as an example....yes they are 904, yes it's an in house movement in the latest versions (and a darn fine one), yes the fit/finish on the latest are really good (not so on the older versions), etc. etc. etc.
HOWever, I've seen some watches frankened by some guy's on here that are right up there with genuine. Plop a genuine movement in them and they would be practically INdistinguishable from genuine. (IF they are distinguishable) There was a thread running on VRF awhile back in which some of the dealers on there were warning guy's how good the frankens were getting. They listed several examples of how hard it was for them to determine if a watch was "fake"....especially with the vintage stuff they deal with a lot. IF those guy's are having a difficult time determining if a watch is genuine at times then imagine the average AD, much less their staff. To me that's the ultimate example how good SOME replicas have become today.
I can tell you this from personal experience. I've owned/own many fine watches over the years, including every SS Rolex Sport model other than the YM and Daytona, and I have SERIOUS reservations about EVER buying another. (unless it was THE "deal" of all deals ) The primary reason (other than the outlandish price tag now days) is I'd feel somewhat ripped off knowing that the Chinese can manufacture and sell a watch with comparable components for $400.00 (and turn a tidy profit) while Rolex wants 7k USA for, more or less, the same SS watch....and yes, I'm MORE than aware about 904 vs. 316, COSC, long term resale, quality of a Rolex movement, etc. etc. etc.
You could also substitute Panerai/Omega and several other brands in place of Rolex.