... Holding documents to strict schemas can be ineffective, because documents change over time. Sure, you can just create more and more tables, but that requires administrators and time....
Suppose the(y) add a field to invoices. And it is required on all future invoices. With RDBMS, you need to create a new table with a NOT NULL constraint.
There are good reasons to choose NoSql (I like my paycheck), but that's not one of them.
The major SQL products have supported "add column" for decades. I implemented add column in the '80s. ISTR that oracle, db/2 and sybase also supported it at the time.