The main limitation of pq as a system is that there is no defined protocol for updates. The system has mostly been used for directory-like data, where occasional batch imports happen, and are enough. Various update mechanisms have existed in the past, but none have been made publicly available.
Another significant limitation (although less systematic) is that ev, the current on-disk data store, has no provision for binary data. One can work with encoded data, but not arbitrary binary data directly.