the belender node graph is indeed very nice. it's even quite powerful in may ways (e.g.animation nodes -- capabilities, that can by used to create even complex node graphs itself in a generative way -- something, that's also possible in pd/gem ), but it also has to be seen as a very proprietary solution with obvious idiosyncrasies, that do not fit very well into other applications. in this respect it looks closer to one of those other arbitrary node graph image processing interfaces, as found in applications, like lightworks and similar.
natron and nuke on the other hand, are really well designed node based compositing environments, which allow you to utilize all the power of an openfx based modular framework in a very appropriate manner. i really like it! it's so much better, than all this other superficial GUI extensions, full of compromises.
but openfx also doesn't come without limitations. it's extremely specialized and doesn't work for anything beside pure image processing (e.g. audio and timecode preserving...). in this respect i still think, that flame and it's sparks plugins show some interesting advantages. it's probably the only system, where no strict line of demarcation is dividing the time line based interaction and node graph based access. both kinds of GUI representation are simple processed by same reactor engine. that's really impressive! -- and i think, it's the way, i would like to see it done in natron just as well.