Not difficult at all. Assuming normal monorail operations and outside of peak rushes like after Wishes or Illuminations, I've always been able to catch the next available monorail with minimal wait either from MK or Epcot.
Actually flexible transportation options is one of the best attributes of staying at Poly. If you're leaving the MK and don't like the look of the line on the Resort monorail line, just hop on the boat (which is shared with GF). If you don't like the boat line but the monorail line is shorter, do that instead. I like having options depending on the time of day, temperature, and crowds.
Separately, walkability from Poly to the TTC (where you catch the Epcot line) is truly an underrated amenity. We spend a lot of time at Epcot and very much appreciate that aspect of staying at Poly.