Recently there has been a fashion for single speed and fixed gear bikes. One of their claimed advantages is that "single speed is more reliable". This is not true, or not completely true.
 
Single speed is more reliable in that there are fewer things to go wrong. This is true. By the same arguement, a bike without tyres will be more reliable as it will never get a puncture. It will be a rubbish bike that will never be ridden.
 
In the real world, where there are hills, singlespeed is not more reliable and can wear out faster than a geared bike. As there is only one gear, going up hills is difficult and the rider will have to stand on the pedals. This puts great strain on all the transmission (cranks, chain and sprocket). If the bike has a few gears and the rider knows how to use them, a hill may be negociated with far less effort and wear on the components.
 
Worse than wear on components is wear on the rider. A chain is simple enough to replace, knee joints are difficult.
 
The singlespeed fad is a reaction against the previous fad, which was ridiculously complicated "mountainbikes" with springs, pivots and far too many gears. As in most cases, the sensible answer lies somewhere in the middle, a few gears (3 to 7 for most people).
 
Almost all new singlespeed bikes or "fixies" lack the ability to take mudguards, which will lead to very frequent replacement of headsets and chains.