Ordinarily counterfeits are easily caught due to the many security features now included in our money. "Successful" counterfeits are quite rare since those security features are so difficult to copy.
I have wondered if the continual altering of US notes is because of North Korea. I think NK’s primary export for some time has been US$100 notes.
A "successful" counterfeit is anything that the crook manages to unload without it being traced back to him. It doesn't matter if the money is detected as soon as his victim takes it to the bank if, by that point, the crook is long gone.
By that standard, U.S. currency has some problems based on the fact that a decent printer could produce a fake that would withstand a quarter-second glance, which is all some money is apt to get in some transactions. In some venues where lots of cash gets passed around (certain swap meets, etc.) it wouldn't be hard to find a vendor who doesn't look too closely at cash; even if one is caught, if one has bought and sold enough merchandise before using the funny money, one would have plausible deniability (e.g. one could claim that one must have been given the fake note by someone else--one would be out the value of the note, but could likely dodge prosecution).
IMHO, currency should include some reflective or iridescent features that are visually obvious from any angle. I have no idea if the new currency does anything like that.