I'm not going to do a huge write up about it because i don't have my books and so i can't double check anything but the short version off the top of my head is that i have one real criticism of the kind of "optimistic nihilism" that you sometimes see floating online, which I guess by extension kind of touches on existentialism as a whole.
Like, the idea is that life is inherently meaningless, and therefore you are free to create your own meaning. Life isn't a test with a right and wrong answer, it's a canvas onto which you paint yourself in any pattern you want. People take a lot of comfort in that, because it's really freeing to imagine that kind of radical freedom to make life mean anything you want.
But like, most of us don't. The idea that you have the freedom to create your own meaning really doesn't apply to a lot of people. It doesn't apply to the poor, or to women, or the disabled, or the incarcerated, or to children who get cancer, or any number of people who simply don't get a choice about what their life looks like. Deciding that my life is about the pursuit of creative self-expression for its own sake is a lot easier if i don't have to worry about paying rent next month. If someone is in prison, telling them "hey, don't worry, I know things seem rough right now, but life actually has no inherent meaning to it, you have the power to create your own meaning" is lying to them.
The reality is that for some people life is short, cruel, and miserable. Some people, it seems, are born only to get sick and die way too young. Others spend their whole life being punished for the sins of society. Some others must live long lives of suffering, pain, and hopelessness to die in despair so that another person who they will never know can live in luxury and abundance.
I'm not trying to shame anyone, or bring anyone down, or dissuade you from anything. I just think it's kind of morally and logically unworkable to say to those people "isn't it great?"