I successfully used Juce w/ 2 different 64-bit minGWs using gcc 4.9.2, one from Stephan T. Lavavej (aka STL, from Microsoft's C++ team), the other from TDM, but had to tweak a number of things in Juce, most importantly the order of some network headers.
Is this worth reporting or is minGW considered too much of a hack?
Sure. If they are simple fixes that don't break the Visual Studio compiler. However, from my experience, the mingw compiler can't be used for Plug-In development as both the VST SDKs and the AAX SDK need plenty of patches to work with mingw.