The “reliability” of a logic gate describes its mean time between failure (MTBF). Digital machines often have millions of logic gates. Also, most digital machines are “optimized” to reduce their cost. The result is that often, the failure of a single logic gate will cause a digital machine to stop working.
It is possible to design machines to be more reliable by using redundant logic which will not malfunction as a result of the failure of any single gate (or even any two, three, or four gates), but this necessarily entails using more components, which raises the financial cost and also usually increases the weight of the machine and may increase the power it consumes.
COST & LOGIC
The cost of a logic gate is crucial, primarily because very many gates are needed to build a computer or other advanced digital system and because the more gates can be used, the more capable and/or fast the machine can be. Since the majority of a digital computer is simply an interconnected network of logic gates.