The Drow's barrier/misdirection thing wasn't a weapon. It was a defense. And it was failing - the Dwarves penetrated it and were actively waging a war against them on the wrong side of it when Cat went into the Everdark.
Liesse probably would have worked.. for a while. The idea after all was to *not use it* - it was to be the sword in the terrible-wine-or-get-stabbed diplomacy. But it also would have attracted or spawned heroes all across Calernia to try and destroy it, and not even Black could fend off the stories that would build against it forever. It would get destroyed sooner or later, and if they were lucky it would be the only thing destroyed when it happened. And then what would happen to the agreements made with the other nations under threat of Hellgate? Or if by chance it actually did get fired? Praes becomes an even worse pariah for having made and threatened other nations with the weapon, or it gets an honest-to-Above Crusade launched on it for having used it. One that is founded on needing to put down a lunatic nation that actually has a hellish weapon, not just political expediency and 'Well, you know, Praes is like Evil and things, so this is Good.'