In the relational world, you can get a lot of insight into what will and won't work with simple math. There certainly are surprises in every database, but most things are not surprises.

NoSQL databases tend not to leverage such a clear and complete model. You have to study each implementation to learn where the surprises are, and there are more of them.

That's not to say they don't both have their role, I just wonder if people who have a little (and in my care very little) CS theory still remembered find RDBMSes easier than those who have learned development without theory.