The claim here is even stronger, and somewhat different: that suffering and reward are fundamentally different kind of things, and cannot simply undo each other

The claim here is even stronger, and somewhat different: that suffering and reward are fundamentally different kind of things, and cannot simply undo each other, not at any exchange rate. The difference is qualitative, not quantitative. For example, if you lose your eyesight, it's such a big loss, that most likely nothing can balance it out ever. It has no "five times as good" equivalent.