Yeah but the title sounds like "you shouldn't just always use emplace_back instead of push_back" but the actual content is "you shouldn't use emplace_back completely incorrectly because you don't know what it is".

The only actual argument against blindly using emplace_back everywhere is that it has worse compile time in a deliberately pathological microbenchmark. Not convincing, though it is interesting.

But then you could write a blog post about pretty much every API misuse ever, which doesn't really seem very constructive to me.

But okay, maybe it's more obvious when having worked with pre-C++11 code, where you would have killed for something like emplace_back. If you are new to the language and only know modern C++, you probably don't think much about why emplace_back exists in addition to push_back, and why it can be faster.