The thickening abilities of flour, cornstarch, tapioca, and arrowroot are almost identical. Flour, however, tends to produce dull and cloudy fillings while fillings thickened with cornstarch, tapioca, and arrowroot remain clear.
Some people object to the tiny gelatinous pearls that appear in the filling of tapioca-thickened pies, but crushing tapioca into a powder—in a spice grinder or with a mortar and pestle—creates a nice clear thickener for pies.