I went on the same trip as Riverstrider.
There is a several hundred yard carry where carts are not useable at the Umbagog end. Then there's roughly three miles of rocky jeep track.
We had two solo canoes and two tandems all loaded with Four days gear and food. All running heavy duty carts with 16" bicycle wheels. The solo boats were easy to zig and zag between the boulders, the tandems less so. One of the tandems crashed, bent a wheel past usable and cracked the hull.
Of course if you are going from Richardson to Umbagog, and if the Rapid is running, and if you are comfortable on Class IV rapids you can just float it.
Metallak Island on Richardson was as good as it gets in a heavy tropical rain storm. But if the cabin was occupied it might feel crowded.
I really liked the campsite at Stoney Batter Point.