I have likewise never had a non-slip loop fail - yet. The number of turns in the clinch portion of the knot is important. I also make sure that the tag end passes back though the overhand knot in the same way it exited after the clinch turns are complete (if that makes sense).
I believe Lefty ran a series of engineering tests to substantiate his claims for the knots strength. I have used this knot to create loop-to-loop tippet connections down to 7X and I have a lot of confidence with it.
Conversely I have had problems with surgeons loops, especially when tying them with fluro in lower tippet strengths. Both the loop and dropper knot seem to be prone to not bedding down completely. When subjected to a sudden jolt, the resutling friction as the knot tightens results in failure. This seems to be particularly acute when joining different diameters of fluro.
I like the surgeons knot for creating droppers but when using fluro I pay extra special attention to tightening both the standing line and tag-ends to make sure everything is solid. Oddly enough, I never had problems with this knot using regular mono down to 3lb test over the past 25 years.
To be fair, there is no substitute for running a series of tests under lab conditions to isolate random variables other than inherent knot strength. I recall using a device called a Hounsfield Tensometer for very accurate tensile strength measurements back in my Physics class. However a comprehensive test would also inlcude some measure of the knots ability to absorb 'shock' loads.
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