Then you just have a system that has no security advantages but is harder to reason about due to the additional level of indirection in overloaded addressing (b

Then you just have a system that has no security advantages but is harder to reason about due to the additional level of indirection in overloaded addressing (because of ambiguous addressing, management of forwarding rules, etc) vs normal firewalling without NAT. Which equates to some loss of security on the system level, because you can only effectively secure systems you understand.