The first thing that comes to mind is that you almost always need to mod them (i.e. remove a filter sponge, replace a pump, add a Hydor-Flo)
That and some models have been prone to cracking (mostly JBJ). As far as I know, few people use skimmers on Nanos, usually just a surface skimmer. A chiller only if it gets really warm where you live, and then there are some small ones that aren't too badly priced.