Should I include powerful formations, and to a lesser extent powerful Heroes, in my armies? The reason I'm cautious to do so is because if you take a strong formation you obviously place a lot of your hopes on it. And then if you have an unlucky turn that formation can vanish from the board, along with all of you battle plans. Think of it this way: if you have a powerful formation it will be the foundation of your whole army; take away the foundation and the rest of your army collapses.

I'm interested in people's opinions on this.

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"There are few left in Middle Earth like Aragorn, son of Arathorn." - Gandalf, Many Meetings

There's no harm in having strong formations, but like in anything, you should never have your plans hinge entirely on a single thing. On top of that, you'll want alternatives and other strong formations as well as a horde of weaker ones.