Storing electronics in low temperatures
Most electronic components are rated to withstand storage temperatures of -25°C or lower, so -30 might be a bit on the low side for a few parts, at least according to the data sheet limits. Electrolytic capacitors and batteries are among the items which may have issues. 

It's usually thermal cycling that causes problems rather than cold temperatures per se. As little as a few thousand thermal cycles can cause problems in some cases. 

Many parts can withstand cooling to 77K (-196°C) or 4K (-269°C) a few times without damage. 

Assembled units (and I think especially with Pb-free solder, which seems to be more brittle than Sn-Pb solder)- I would be particularly wary of extreme cold with units incorporating BGAs, just because of my personal experience with failures and due to the difference in CTE between ceramic and FR4 PCB materials. TQFP and other packages have a bit more 'give' (available compliance motion) in that the leads can bend a bit.