> your heuristic doesn't work on my carefully constructed pathological case

This is a bit like writing unit tests, debugging code, and so on.

It's a heuristic (claims to be probably helpful). You might as well complain that checking string size first as part of an equality check of strings that demonstrably have a very large distribution of sizes is obviously dumb because obviously you can construct same-length strings at will. Breaking rules of thumb is the easiest thing in the world.