Amsoil could be the best thing since sliced bread, but I don't really care. I don't use it. Actually, I have always used inexpensive brands of TCW3 oil in my outboard. Walmart or Mills Fleet Farm brands. They work just fine, and I've never heard of an engine failure caused by them, either.
My objection is with their 100:1 mixture recommendations. If their oil is TCW3 and I had some, I'd use it, mixing it according to the outboard manufacturer's recommendation. If it doesn't have TCW3 certification, then it simply wouldn't go in my outboard.
I have no idea whether it has that certification or not, but I've never seen it on the shelves of the places I buy outboard oil. So, it's really a non-issue for me.
You want someone to show you an engine failure caused by Amsoil. Good luck with that. You could ask to be shown an engine failure caused by Mercury's oil, and you'd get the same response.
Seems like much ado about nothing to me. A guy who uses TCW3 certified oil, in the ratio recommended by the outboard manufacturer is unlikely to ever have a oil-caused failure. So...why would anyone do anything else?
Why would you? It just doesn't make any sense to me.
One thing I can tell you for certain: Any time someone on iboats.com forums says that people should use 100:1 fuel

il mix in their outboard, you'll find me telling them they're wrong to recommend that. It could void their warranty if they have a newer outboard, and just isn't safe in other motors. You'll see me saying the same thing when people say to run 50:1 in outboards that require 24:1.
Oil brands don't matter all that much, as long as the oil meets the certification standards. The ratio does matter.