Finally, I would like to add that I feel like a one-man band against an army of opponents. I don't appear to have many supporters. Or, am I just being over-sensitive?
Well, not entirely. I don't feel that Tetra SafeStart actually makes a tank safe for fish immediately, but it can definitely jump-start a "cycle", and having a "cycled" filter does appear to prevent "new tank syndrome" in my experience- if I have a "cycled" filter, I can put it into a brand new tank and stock it immediately without losing fish to ammonia poisoning.
I use sponge filtration exclusively in all of my tanks (one has a hybrid HMF, but that's just a fancy name for "gigantic sponge filter"). My procedure when I get a new sponge filter is to fishless cycle it in a 20l bucket with Tetra SafeStart and ammonia. When the cycle completes- determined via the test kit method that is disliked here- I then add it to a tank, and have never had a problem with stocking the tank straightaway. I used to do fishless cycling with ammonia in planted tanks, but after suffering a massive melt of about €100 of anubias, I realised that cycling with ammonia is pretty rough on plants.
When I got into planted tanks, I was first enamoured of the idea of going filterless, but it seemed safer to have the insurance that a sponge filter provides. I have never lost a fish to ammonia poisoning when using this method.
I get what Darrel says about the filter being a single point of failure, but I don't feel that it applies here, as the filters all go into planted tanks. They are part of a system that includes the plants and the substrate, and that system works beautifully for me.
Also, when I used Tetra SafeStart, the process took about three weeks. I just finished one in which I used Seachem Stability, and it took more than six weeks (but it cycled eventually). Now I have a much better understanding of why this recent one took so long to finish!