Low cohesion adds complexity which can translate into a reduction in application reliability. The more highly factored an inheritance hierarchy is the greater degree to which method reuse is likely to occur. Highly factored applications are likely to be more reliable for reasons similar to those which argue that such applications are more maintainable. Greater method complexity is likely to lead to a lower degree of overall application comprehensibility. Greater method complexity is likely to adversely affect application reliability.
More complex methods are likely to be more difficult to test.
One of the goals of object-oriented design is finer granularity. Therefore, each object's behavior should be more easily understood and analyzed.