I agree that probiotics, in general are fragile. In addition, I also agree that the highest effective colonization is most likely further down in the small intestine into the colon...
This based upon the location of the appendix and its little understood function in human adaptation\evolution to include the recolonization of bacteria after an illness or, the eating of a poor food which causes intestinal distress...
The location of the appendix, its function, and the location of the ileocecal valve are most likely some clues here...