This probably belongs more in tactics, but it's worth noting that FC puts your models up a step on Initiative, helping you out on combat resolution the turn you charge. Now, your Ancient is going at I6.
It also can affect characters like Logan (if he goes at I1), Draigo, Marneus, and Dorn, and does some good work against Tyranid MCs (although DE don't need much help against them). I've used it before for my Grotesques, and it works well against Librarians (when charging with FC) to protect against the Force Weapon.
I have used Urien in a list, and his biggest weakness is S10 or other ID weapons. However, most of these will be going at I4 or lower, giving you a good chance of avoiding them.
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