Most units aren't worth building once significant numbers of their higher-tech successors are on the field. Medium tanks are obsoleted by heavy tanks, which are in turn outclassed by siege bots or similar.

I'm aware of a few exceptions, though. For example, my understanding is that frigates make a cost-effective meat shield long after destroyers and battleships enter the fray. I've also heard that interceptors will beat T2 fighter/bombers cost-for-cost (though I'm guessing not swiftwinds or ASFs, right?).

What other units share this property of remaining unexpectedly viable above their tech level, and why?