Evert Pot has shared some of his thoughts about why he has a problem with password_hash (and friends). His thoughts are initially about this particular feature but they're actually wider than that.
The initial introduction and rfc for these functions made me uneasy, and I felt like a lone voice against many in that I thought something bad was happening. I felt that they should not be added to the PHP engine. I think that we should not extend the PHP engine, when it's possible to write the same API in userland, or there are significant benefits to do it in PHP, such as performance. Since the heavy lifting of the password functions is done by underlying libraries that are already exposed to userland-PHP, it didn't make sense to me to expose it as well in the core.
He includes a list of things he sees as drawbacks for new C-based functionality in PHP including the fact that it extends the "PHP specification" and forces other projects to implement it (like HHVM). He does include a few positives, though, such as the increased visibility and legitimacy, but still thinks they don't outweigh the negatives.