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...
If you are working with a SD card, sometimes when you want to do something, it simple doesn't do it. In that case i have to flash the bootloader again.
For example, saving values inside variables, comparing values from one variable to another, it doesn't work.
The magic about the counterfeits is that if you flash the bootloader, they start to work again (well at least it's how i've been solving mines)