The real issue is the formula (BTW, I should comment that the work Andrew, Hunter Allen, and others have done with these training metrics is fantastic; I don't want to seem too critical) is that it isn't linear with time. Neglect the 30 second average. If I do an hour at 80%, that's a TSS of 0.64. If I immediately do another hour @ 80%, the TSS is doubled, to 1.28. All is well: the TSS of the total = the sum of the TSS of the parts.
But once the power is different in different segments, this no longer applies. The total no longer equals the sum of its parts. You can't do an hour @ 80% (TSS = 0.64) + an hour @ 50% (TSS = 0.25) and get TSS = 0.89. No -- instead the TSS of that workout is 0.97. There's an "extra TSS" component which leaks in, as a result of the formula.
A result of this is adding micro-power or zero-power segments to a workout can add TSS. This creates non-intuitive results. Like if I finish the workout, then take a nap, the TSS is less than if I take the nap in the middle
. This is a pathological case (I assume!). But if we both do a century ride, and you spend an extra 2 hours at rest stops, should your TSS be higher, if we ride at the same power, and have the same FTP?
I don't doubt descending adds stress on many roads. So does weather, but weather isn't included. TSS works great, in practice, I'm sure. But it is not necessarily unique in this regard. It is possible it has potential improvement. My guess is a linear-time formula might better capture what is intended, even if TSS is already way better than miles, hours, or kJ.
Dan