I don't feel like I learned anything new from this book. A lot of this is common knowledge. It also didn't hold my attention so I could have missed some details. The author acknowledges that he didn't raise his kids like this. 
Also he referenced the book Free-Range Kids often. His family grew up in New York City.  I'm from the suburbs so it seems bold to give kids that kind of freedom. But I know in other countries children are given more autonomy earlier on. That is one good thing about this book. It's a different perspective and Haidt was able to talk about the anxiety he felt letting his kid be more independent.

More opinion based than science based despite how he sets up his arguments. Often cites his own work which he mentions are often conclusions he simply reasoned without hard proof since he can’t imagine an alternative.  

Also references an uptick in trans children and those with gender dysphoria as a result of the rise of social media, etc. Greatly over simplifies many topics such as these then simply moves on versus talking about the countless other circumstances that could be part of the bigger picture or the support those children also need. 

Do I agree that earlier use of phones and social media, as well as overall screen time is problematic, yes. 
Do I agree with all of his assumptions, absolutely not.
Problematic that so many assumptions are presented as fact.