I'm coming to think that the main advantage for using static isometric contraction to build finger strength (rather than the well-proven dynamic concentric / eccentric contractions used for most other muscles) is that it's consistently and accurately measurable.
So if you're not getting that benefit from static hanging, you might as well be doing some dynamic finger exercise with resistance.
. . . A pulley system (instead of resistance band) makes it simple to get consistent accurate measurements, and simple to quickly change the resistance force in a well-controlled way.
So why not focus on dynamic finger exercise?
I think the big problem with accurately measuring both the resistance and work load with a dynamic contraction exercise is that the motion of the finger joints is so small that it's really hard to know if the start position and finish position are (even roughly) the same with each repetition -- first because it's just difficult to see it carefully - (and perhaps also because the percentage in change of position due to varying thickness of skin and related soft tissue under pressure becomes significant when the range-of-motion is that small).
Even more so to measure (or at least control reliably) the range-of-motion distance between different sets in a session - (never mind between different days). So even if you accurately know the resistance / force on each (dynamic) contraction, you might be getting that force through a different range-of-motion distance. Since range-of-motion distance is a key factor for determining the Work load of each (dynamic) repetition (and also the Power rate), then you don't reliably know what your Work or Power output really was.
So like perhaps you might be slightly increasing the resistance force (accurately measured by your pulley system or simply added weight in a belt or vest), but at the same time (unconsciously?) decreasing your range-of-motion distance (not well measured). You might then believe you're getting stronger, but really there's no change (or even a decrease?) in Work output per repetition.
But if you can't consistently accurately measure your Work load progress in a dynamic special training motion, likely to get better results for climbing strength by just performing actual climbing motions (which you're not measuring accurately either).
Summing up . . .
1. Strength + Hypertrophy of finger/forearm muscles and tendons is way critical for improving climbing performance.
2. For most muscles and tendons used in most sports, static isometric contractions are an inferior means of training strength + hypertrophy.
Concentric / eccentric contractions achieve faster bigger gains, and both resistance force and range-of-motion distance for those can be measured (or reliably repeatedly controlled) for the larger motions of larger muscles.
3. But for fingers the (small) range-of-motion distance is not reliably repeatedly well-controlled.
4. Therefore static isometric training makes sense for the special case of finger/forearm muscles + tendons for climbers.
Anyway climbers also get lots of valuable dynamic concentric/eccentric contraction stress on their finger/forearm muscles + tendons -- from performing actual climbing moves near their limit.
. . . Just thinking (too much) . . .
Ken