They can be. I have all sorts and never had a problem, but I've avoided the absolute rock bottom cheapest Chinese ones. Since it's all OSH... I always work in Windows so I've not had a problem with the clones that use the CH-whatever USB chip, they've all just worked. Some of the Nanos I have look a little dodgy in the assembly and soldering department, but I guess I can't expect much when I paid less for each of those than I can get an ATMega328 bare chip for. So far every one from the 10 pack I bought has worked - it was a more expensive one that had a bad solder joint on a capacitor which I only made worse in an attempt to repair - SMD and my poor vision do not mix, lighted magnifier and microscope included. I really don't know what I am going to do with 10 Nanos, other than, at a cost of about 1USD each, not worry about accidentally frying a pin or two while prototyping things.