sometimes they have problems with simple calculations
Can you point to an actual example of that?
Really bad counterfeits probably won't work at all, or have a different chip than advertised (lately there have been several instances of people ordering Atmega328-based boards, and getting a board with an ATmega8.) And there are boards with counterfeit USB chips (prolific being widely counterfeited, in particular) that may have issues with USB drivers. But I've never heard of a "partially functional" counterfeit giving bad results to calculations...