Sure, because the BBEG is sitting there for rounds just allowing you to cast those spells. You have to roll badly enough on the portent rolls to cause the BBEG to fail two saves. He has to not have abilities to counter those abilities. With that kind of effort, sure, go for it. At that point you are high level anyway.
As for the 1 in 6 chance of it being cast on you in a campaign, that translates into once every 6 campaigns, which is right in with what I said, AND the druid has to fail the save. Players just don't have to worry about it.
It's just one spell, polymorph and that's it. If he fails it (portent would more than likely make that a heck yes) he is dead. He can't do anything but make a slow slime trail as a slug while you load up disintegrate. That's not just a druid, that's casting poly on just about anything.
And the 1 in 6 thing isn't campaigns, its if you have a druid over lvl 10. When you play over 10, someone eventually gets hit with that spell, always. If you have 6 people in your party, 1 is a druid, 1 in 6 of him getting blasted. If no druid, then 0% chance.