Final answer:
The primary disadvantage of structure type equivalence is that it can result in incorrect assignments or comparisons between structurally differing types leading to potential bugs in the code.
Step-by-step explanation:
The primary disadvantage of structure type equivalence in programming is that it can lead to incorrect assignments or comparisons between two structures, particularly when the structures have the same sequence but opposite configurations. Specifically, if a sequence of structure fields (for example, from field #1 to #2 to #3) is used to compare two structurally different types, it can yield an incorrect result. This is due to structure type equivalence considering two types to be the same if they have the same structure, without taking the actual configuration or semantics into account. Therefore, operations that rely on the structural sequence may incorrectly assume that the two types are interchangeable, which could introduce bugs into the code.