Apparently, they are bad because JOINS are not really set operations, but are actually a cartesian product - https://blog.jooq.org/2016/07/05/say-no-to-venn-diagrams-when-explaining-joins/.
IMO, that might be technically correct. But, to me, the venn diagrams are easier to understand and remember compared to cross product diagrams. I don't know why venn feels easier though. Maybe because venn is the first thing I learned and got used to it, or maybe it is really easy because the diagrams are simple. IMO, what matters is whether something helps you to understand things easily and correctly. Does not matter if it is "technically incorrect".
I think that it's valid to whatever extent the analogy helps you improve your understanding. If you don't know where the analogy ends and ceases to be applicable, then you might have a problem where it starts to mislead rather than clarify.
The best ideas are the crazy ones. If you have a crazy idea and it works, it's really valuable.—Kent Beck