Well, sure, but most supernaturals know exactly what they are, and many of them are likely immune to manipulation...
Nobody is immune to manipulation.
Yeah, all that is true. But -- before they could get access to modern weaponry -- what would protect them from someone (Black Court, Red Court, White Council...) simply kicking in the door at a big White Court meeting and wiping out most of the leadership. The uberghouls were pretty effective at that in WN, and I think a fairly small strike force of Wardens or powerful Black Court vampires could accomplish pretty much the same thing. So how have they lasted so long?
You're wondering why large and fractious organizations in pre-modern times didn't assemble an elite strike force to assassinate the leadership of a friendly-ish faction in an unprovoked illegal surprise attack on a meeting they probably don't know about which is taking place in a location they probably can't find, knowing that their organization is almost certainly being infiltrated and manipulated by their targets?
Uhuh.
1) Gather many wizards in Edinburg.
2) Put a couple hundred wizards into putting their reserves into a ritual for a few months. (i.e. offering consequences/declarations every so often)
3) Have the Merlin cast the 100.000 shift ritual into a generational "curse" on Humanity; a permanent ward against being fed upon by the White Court.
4) White Court starves out.
Probably wouldn't work, unfortunately. The GM would interfere. Game's no fun if you just let the players win.
The exact method of interference will vary by GM, of course. I personally would probably say there's at least one enthralled wizard who deliberately sabotages the spell, causing massive fallout/backlash. Unless the players find them and stop them, of course.
And halfway through the ritual preparation an interpersonal conflict between two wizards with severe personal disagreements turns bloody. Other wizards take sides.
Also, someone tells the white court about this in advance and they do something clever in response.
And then humanity dies out because wards don't move, and with the ward in place everyone is paralysed.
Okay, I'm kidding about that last one. But honestly, the rules foundation here is dubious.
More to the point, the game and the world are not the same.