I meant it doesn't have a decimal type but looks like as v3.4 it does: https://docs.mongodb.com/manual/tutorial/model-monetary-data...

Ok, but even prior to 3.4, in what sense did you mean it couldn't guarantee something was a number at all?

Not having an enforced scheme means that there is no guarantee a given key exists or that if it does exist, the data type matches what you'd expect it to be. One errant piece of code could insert a bad record that could break some other of piece of code.

Since version 3.2 you've been able to enforce data types with validation.