The fact that it's based on set theory is utterly irrelevant. The question is: what is the best database from an engineering perspective? It has never been proven that RDBMS's are the best in any respect whatsoever in this sense.

It's irrelevant that you can concoct an arbitrary mathematical theory and then create a system to match. You must also prove that that theory is the most relevant and important way of looking at the data for the given problem at hand. RDBMS theory doesn't even bother trying to worry about the real world.