This is a short review of Jonathan Haidt’s book “The Coddling of the American Mind”. [I do not know if Haidt is good for liberalism.]
Microaggressions are intentional or even unintentional slights that “communicate hostile, derogatory, or negative attitudes toward stigmatized or culturally marginalized groups”. The slight can be something as seemingly innocuous as “What country are we from?” Undoubtedly, this comes across as absurd. But if we put ourselves in the shoes of others who are inundated with references to them belonging to a marginalized group, then we could see this as an innuendo. The standards for what is abusive have rightly changed from physical abuse to emotional abuse, which is rightly defined as whatever is subjectively traumatic to an individual.
Jonathan Haidt has written on microaggressions and the morality behind politics. Although he is correct on us having evolved psychological adaptations that make us sensitive to topics of fairness etc., his advice on microaggressions seems to be out of his field of expertise. I agree that if we don’t have unstable self-esteem and a history of abuse that it is better to learn how to cope with insults versus avoiding them. Most people can learn how to not personalize the message. Haidt is mistaken though when discussing how we should approach microaggressions in that we should always give a person the benefit of the doubt when assessing their intentions over a perceived slight. There are circumstances where people become the target of ridicule and bullying.
Haidt’s central claim is that upon exposure we become desensitized to insults, but he fails to mention that we can also become sensitized. Researchers do not know what circumstances lead to which. Haidt is thus wrong to say that what does not kill us makes us all stronger. People vary in their resiliency. Granted his audience is college students who are probably only at minor risk for interpersonal bullying and rejection, he seems to generalize this to anyone who gets insulted. Political correctness and popular exposure has helped in improving the status of women and LGB. But there are many who are still rejected and ridiculed because of a disability, gender, or physical deformity. If Haidt means to exclude extreme cases in his analysis, he sure is not clear about it.
What Doesn’t Kill Us Makes Us Stronger?
But teaching kids that failures, insults, and painful experiences will do lasting damage is harmful in and of itself. Human beings need physical and mental challenges and stressors or we deteriorate. 
I have not looked at the evidence that Haidt has for his views that exposure to insults and failures are necessary to prevent mental deterioration because there probably is none. He argues by way of analogy and gives examples of how resilient the immune system and skeletal muscle are. The point is that we need to stress these systems in order for them to grow. The human mind is different than the immune system and muscle though. The mind is very sensitive to glucocorticoids which are released when we are threatened or hurt by insults and criticisms. In fact, some of the most potent causes of cortisol being released come from negative interpersonal interactions. Haidt’s analysis is too generic; he does not take into account the severity and occurrence of insults.
Haidt’s argument is based on the success of ERP or Exposure and Response Prevention therapy. The premise is that we can desensitize ourselves from our trauma and fears by exposing ourselves to them. If, for example, we give into our social anxiety and do not go out with our friends, then we are reinforcing the fear, making it easier to avoid instead of engaging. But I am not aware of any studies that look at how exposure to criticism and insults can desensitize us to the feelings of inferiority and worthlessness. Even if we could become desensitized, this would not work for everyone since people vary in how fragile or resilient they are (i).
There is an abundance of evidence that suggests that early peer rejection and bullying predispose an individual to anxiety and depression. Marginalized groups, which not only include race, ethnicity, sexual preference, and identity but also those deemed as inadequate and undesirable, are more likely to be rejected and bullied. The question becomes is safeguarding our mentally healthy youth from microaggressions a strategy that will help or harm them in the reality that we cannot abolish them. This is the only part that I’m in agreement that it is more effective to teach youth how to cope with criticism and insults than to safeguard them. But this cannot apply to those that are routinely bullied or dismissed because constant criticism is documented to cause subordination (ii).
The rest of Haidt’s analysis is overreaching his field of expertise, which is clearly not within the area of psychopathology. Although I have not looked at his evidence, I am very suspect of the claim that providing safeguards in universities and colleges are contributing to the increased rates of depression and anxiety. Haidt believes that we can overcome trauma and become better people as a result. I agree. But marginalized groups can face constant levels of belittling in which the fight or flight system breaks down and can cause depression. When Haidt says that “what doesn’t kill you makes you stronger”, this is incorrect. Whether or not we are resilient or become subordinated in the face of belittling is based on our dispositions and how frequent and severe they are.
i) One group which Haidt ignores is 20% of the population that has the trait of sensory processing sensitivity. They are known as Highly Sensitive People. They are sensitive to subtleties and are more easily overwhelmed than most. In fact, skin conductance tests, SPECT, and functional MRI tests reveal differences in reactivity. This group may be served better to be protected than exposed to microaggressions. This is especially true if they are easily rejected based on possessing stigmatized attributes.
ii) If one was interested, I can furnish tons of references on how pervasive criticism can lead to depressed mood states and anxiety. When people are consistently disparaged, then they become in a defeated state. There is a whole body of evidence on how depression is the result of spousal criticism and how early peer rejection predisposes one to anxiety and depression.
 Lukianoff, Greg and Haidt, Jonathan. The Coddling of the American Mind.