The armies can’t be too good, they get sent against you too. The game would never progress if they were an automatic stop ritual button.
For an interesting data point, in my current High Elf (Lothern, Normal) game, I sent out a 10k intervention force against Naggarond for the second ritual. Not only did they raze one of the ritual sites and prevent the ritual completing. They went on to raze six (6!) other Naggarond settlements. And they’re still going.
But there’s nothing random going on here, this is all strategy. Naggarond had sent their main army and their black ark along the the northern edge of map, apparently attempting to take the Shrine of Khaine before I could get to it. Didn’t work out too well for them. But it’s not random.