yeah, the UI is a large part of the issue - but definitely not 100%
its picky, e.g. you have to use triggers for multiple outputs, since the order of messages sent is defined by the order you connected - that's just crazy.
there are so many 'versions', pd-vanilla is fine, but then there is pd-ork, pd-extended... and now these will become defunct, to be replaced by deken... perhaps! its just confused, and lacks direction.
the UI patch elements are pretty poor/primitive.
it doesn't support (as far as I can tell) a proper polyphonic model ( like max poly~), copying encapsulations, and linking to poly is a horrible workaround.
it doesn't support (again as far as I know) , multiple threads (max achieves this, also via poly~), this is pretty important for low powered processors like the rPI which have multiple cores, but each is not that powerful.
frankly, it just feels like a mess...
as for Max/PD, I don't think Miller has anything to do with Max these days, basically he created it, then forked PD off it, then sold Max to Cycling74... who in turn have recently sold it to Ableton.
PD just feels like its never really moved on from than fork, where as Max has thrived as a commercial product, not only on the audio side, but adding graphics and much more (e.g gen/m4l)
I keep coming back to PD because of lack of choice, and hoping if I get used to it, I might find out why others like it... but if there was Max or Axoloti on ARM, Id drop PD in an instant.