> With mongo, you just add a column and you are done.

No, usually you are not done. Your data doesn't have the column. So either you write your code in a way that it can handle the column being optional or you write code to read each document and add the column manually.

I've started out with mongodb on Fit Analytics originally, until we noticed at some point that our data was an inconsistent mess and we'd implement constraints in our back-end code to make sure the data is consistent. Eventually it became just easier to have an SQL database taking care of all those constraints, and we luckily migrated away.