Since you have tagged this with "sociology," I assume that you are talking about the process of using knowledge to achieve greater control of the societies in which we live.
The major advantage of rationalization is that it can make society more efficient and fair. A society with rationalized laws and procedures can act more efficiently than one that operates on an ad hoc basis. When laws are implemented rationally, people in similar circumstances are treated in the same ways. This is fairer than a less rational system.
However, there are disadvantages. A rationalized system is one that is rule-bound and bureaucratized. People who are simply applying rules often have a hard time seeing the differences between individual cases and applying rules in ways that cater to the needs of the individuals rather than to the letter of the rules.
A rationalized society, then, is more efficient and fair, but it can also be inhumane and impersonal.
The problem of the increased rationalization of society is referred to as the Weberian paradox after sociologist Max Weber (who developed theories about the rationalization of society). The increased rationalization of society made people less tied to the strictures of traditional society, but, at the same time, it made them beholden to the structures and rules of bureaucracy.
By developing universal and abstract rules about the way in which society is governed, bureaucracy can lead to greater efficiency and precision. For example, everyone is subject to parking tickets for parking in a no-parking zone, and the traffic cop does not need to examine the reasons why the person has parked in that zone before giving the motorist a ticket. In other words, formal rules make the administration of justice easier and more efficient, and these rules make the functioning of the society more orderly and predictable. However, the rationalization of society means that rules and bureaucratic structures can become excessive and not allow for ethics. For example, what if someone parked in a no-parking zone because they were bringing a very critically ill patient to the hospital? Should that be a reason for the person not to receive the same consequences as someone who parked there out of sheer laziness? In other words, bureaucracy results in dehumanization and imposes rules without always giving regard to the reasons for human behavior.