dhclient not having support for IPv6 prefix delegation size is a major showstopper for many users, e.g. Comcast requires it if you want to delegate internal IPv6 networks. See bug 876791.
This feature is now supported in DHCP upstream as of 4.4.0; the current upstream is 4.4.1, but none of the Fedora builds currently have more than 4.3.6.
*** This bug has been marked as a duplicate of bug 1550294 ***