We know that trying to hold someone using the opposite sex's Power is already at a disadvantage versus holding someone of the same sex. It can merely be the case that while it is more difficult for a woman to hold a man, requiring more of them, it is not actually indicative of that vague state of what "strength" means as we have been using the term. We've seen Semirhage held by relatively few Sisters, but this doesn't prove how hard it is for a top tier male to be held by other males - apparently, they don't even try most of the time since they can't Link to boost the effect of the Shield. Being that none of the male Forsaken outright Shields any of the female Forsaken, it might simply be the case that they don't do it because the results are not guaranteed as it could be that the difference in the Powers makes it harder for them to hold such a strong female while doing what it takes to kill her. I'm pretty sure the way RJ uses "strength" as a term can simultaneously be both subtle and too blunt to get at what we see occur in the series.