Just to chip in on my experience on this matter. I use UPX as one of the RE defense method on a couple of Delphi based software we build which our customer runs regularly on their servers. One of the challenge is some A/V throws a false positive upon checking the result files. Somehow this became no longer an issue after applying code-signing to the UPX output executables.
How does UPX defend against reverse engineering? The binary literally contains the code to reverse the UPX compression (otherwise it couldn't run), and I'd expect all antiviruses to be able to unpack UPX executables.
