This is referring more to schema than data. In part what that means is to avoid nested indexes... subtle but different than avoiding any nesting at all. In other words, if you can treat the nested data as a blob, it's probably okay, but if it's being used for a query, it's adding complexity that can cause trouble.

Some of the reasons for that are Firebase-specific, it has to do with security rules and how security can get too complicated if you're not careful with nesting.

But I'd guess it still applies to other NoSQL data... nesting data as part of the schema is like making another table, and all the complexity that comes with it. Except it's a new table you can only get to by going through the first table.

A common problem with nesting is thinking you got the order right for your use case and later finding out you sometimes want to index by the inner data rather than the outer data. If you only have A/B (B nested in A) and you need to query for As, then you're fine. When you find out you need to query for Bs, you have a problem.

Firebase even recommends duplicating data, if necessary, to have two indexes A/B and B/A, rather than trying to query for nested data.