Quantum mechanical effects become dominant at the this scale, thus giving us a chance to study new and exciting physics. However, this also means we don't know enough to take the full advantage of the new effects; and these new effects will work against us until we understand them.

As we shink a device, the technical difficulty of fabrication increases, making the process less reliable. What's worst, since we also want to put more components in the same device, the probability of getting a fault component in a device increases dramatically. These are experimental and technical challegences.