dry boulders, your traction on a sloping surface is NOT as good with felt as it is with various kinds of spider rubber.  The felt or carpet fibers probably begin to slip and roll at higher angles.

When wading, swimming, and rock-hopping the Chattooga Headwaters, I found felt soles (actually carpet I glued on the soles) to be absolutely essential for the wading and wet rock part.  I really could not have made such good, secure progress in spider rubber whatevers.

I have some Chota booties with felt inserts, and they have worked well.  The inserts deal with slimey underwater stuff, and the rubber periphery handles dry rocks.  I have Patagonia felt sole boots, but have not used them. In size 15 they are kinda clunky.

Haven't seen felted sandals, but check the fishing part of the Chota site, and also the Cabelo catalog.