We'll have to agree to disagree. Writing a manual security layer that doesn't understand your data model is one of the worst things you can do. Multiple databases? Now you're also manually doing integrity and probably manually doing joins. That's a ton of extra things to go wrong.

As an example, consider a query that is fairly complex. How exactly are you going to make resuable security tests for it? How do you know that an error in the query won't expose unexpected data, or that a modification to the query that introduces and error exposes unexpected data? Your security now has to be customized to every query like this.

The alternative case is that you only use super simple queries so that you can write resuable security checks. But now I'm going to ask why you are using a relational database in the first place? Because almost certainly you're doing manual joins in this scenario and incurring huge performance hits.

If you are using a relational database to its full extent, there is nothing as secure as built in row level security. You wouldn't dream of doing integrity checks outside the database, why do security?