But it seams that there is really no uniformity between thread strengths and weaknesses.
That's not a flaw of the chart, though. If anything, that's precisely the point it's making.
I have the same issue with thread classifications as I do with fly rod/line weights: there's no simple, elegant way to meaningfully
compare two products, because there's too many forces at play. The measures that are easy to apply across the board (denier, "Xwt" designations) are also often the most useless. Even if we break down and define what exactly we're measuring, it doesn't really serve as a valid comparison to anything other than itself.
So you determine the denier, as a common denominator, of two tying threads, what does that tell you? Nothing really, other than weight.
What does the weight of a fly rod tell you about how it'll fish? Nothing. It's just a recommendation of the line weights the manufacturer feels it'll cast best. And those reference the AFTMA standards...which are based on???
Ultimately, in these things, I think the only really truly valid method of comparison is personal experience. Even two tyers or anglers will have different experiences with identical products. All you can do is try a spool and see how it goes. If you have a strong negative experience, maybe give it a second try, and if it repeats, move on.
Also keep in mind for thread that there's a lot to be said for tying skill, and not dragging the thread across the hook point. I'm careful, but I usually bang the thread off the point once or twice per dozen flies, and with UTC, that usually means fraying if not breakage. Uni will sometimes break, but more often than not, I can save it. That, combined with Uni's bonded nature, gets it the nod 9/10 times for me. Only where thread bulk and smooth bodies are a primary concern do I bust out the UTC 70.