They can certainly be hit or miss. My buddy had acquired a bunch of white xanax bars, that were clearly bunk, or severely under-dosed... They didn't even look right, like the edges were squares off and they were a bit fatter and bigger than the legit american white bars. The ones with "G G '' and a couple numbers. I got no effect from them, but others claimed if they took a few they would feel good.
On the other hand, i had another friend who would get xanax shipped in from somewhere in South America, for ridiculously cheap, via internet which were just fine. With these I noticed no difference in potency from American xanax bars.
Just like any other drug, you can get good shit, and you can get bad shit. All depends on your supplier.