Having problems with 577 packages due to the new version of the GNU Compiler Collection is much more than usual with Red Hat's Marek Polacek pointing out last year with the GCC 5 rebuild they had problems with half as many -- 236 problematic packages.
A lot of the problems this year were due to the default C++ standard of the compiler changing from GNU++98 to GNU++14, with many packages not prepared for C++11/C++14 changes. Changes in libstdc++ also "often revealed very poor programming practices" according to Polacek.
Rebuilding the thousands of Fedora packages under GCC 6 did also uncover some GCC and libstdc++ bugs, most of which are now fixed in the upstream code. Other problems included some packages failing due to the new warnings of GCC6, and other problems like a glibc bug, poor version checks around GCC6, and other problems.
Those wishing to find out more about the Fedora Rawhide rebuild with GCC 6 can find all of the details via the Fedora devel list.