In other words, depending on the degree to which the metaphor is conventionalized in the language, it is expected to be far more difficult to detect using computational methods, since its domain essentially becomes indistinguishable from the rest of the context.
a metaphor such as "boiling-hot anger" is far more salient and would be expected to be easily detected by a context-based metaphor classification system, since the source domain (TEMPERATURE) is very distinguishable from the target domain (EMOTION).