I recently made a toprope anchor extending from a tree about 15' over the edge of the cliff using 1" webbing. The webbing was looped around the tree using a figure 8 follow-through, with sufficient tail and a simple stopper knot as well. A figure-8 on a bight was tied at the master point (opp opposed lockers), where the anchor joined a sling from the single good bolt (the other was loose), the rest of the webbing simply coiled up at the top of the cliff. See the fantastic attached image.
My question is: People seem to think poorly of figure-eights with flat webbing, but in this setup they seemed perfectly fine and I don't see why one wouldn't use them (other than, perhaps, simplicity in using other knots). Is this anchor safe with the figure-8s? Should they be tied differently?