I've only had the the toecaps slip once after 10 days on them, and I'm pretty sure it was my own fault from being careless and not putting them on properly. They work way better than I would've imagined, even on the smooth, rounded toebox of these NB x 686 boots.
I think it has to do with the anchor points of the toe cap ladders. On Flux and Rome bindings there are two anchor points for the toe cap ladders. If the boots are on the smaller end of the size range, you can move the anchor points further back; so the toe cap molds around the boot better.
This is a minor point, as the King Pin system reduces the effort needed to transition to heel side; i.e. I can position the toe cap over my toes (like a toe strap) but use less pressure to secure my boot.