It's maybe not spelled out, but basically already shown in the books that distance is obviously a factor regarding the deaths of Be'lal, Asmo or other Forsaken and their possible transmigrated. You further indicated that this is also supported by RJ's comment about SG and the DO's powers there. But RJ and BS presicely said that there are certain limits, which they won't reveal yet. Therefore I think it must be something different.

And if a major defeat of the Shadow would be the "where-factor", then it doesn't play any role when it happened? That means it doesn't play a role that the defeat was just suffered at the end of TFoH, but that the DO would always have a harder time to transmigrate people in Caemlyn, because he simply suffered a defeat there, irrespective how much time has passed? Seems rather unlikely, IMO. You even implied that "when" would be a factor (not only how and where), when you stated in your previous reply that Asmo "died in Caemlyn just after the Shadow had suffered a major defeat there (namely, Rand taking control of the city from Rahvin after killing him). I think that also was a factor." [emphasis mine]