I think that might be in part due to inexperience tanking as a WAR or just poor positional awareness on the part of those tanks you see in DF pulling enemies out of AoE circles. There certainly is a learning curve to WAR, even gathering AoE hate using a cone attack versus being used to a self-radiated circular AoE like Flash or Unleash if you're used to tanking as PLD or DRK. In short, how you pull may be more of a factor in how much you're moving mobs than Overpower's cone-shape?

I do something I call "tightening the knot" where I run in and straight through a group of mobs instead of stopping at them. Run straight at the center of a group, pop Overpower to hit all or most of the group as you're running at them, run straight through the group after aggro, kite the group a few steps more so they from a tight cluster behind you, turn and hit them with another Overpower. This should hit all gathered mobs. If it doesn't, take a few slow steps back, not enough to move the whole group or pull them outside of an AoE circle, but enough to cause the mobs in the back or periphery to group in tighter. As Overpower is a cone, the breadth of its range is narrowest the closer you are to your group, so if you can stand at max melee range, you're more likely to hit the entire group. Obviously, make the effort to not pull out of AoE circles unless you have to dodge a big AoE yourself and you can't run straight through the mob group and then back to your original position again.

If you've pulled so many mobs that you cannot physically hit all of them with one Overpower, divide the group in half in your head, target a mob in the center of one half and Overpower. Then target a mob central to the other half and Overpower. Likely mobs in the center will get hit by both. Or just hit straight up the center with Overpower and focus single target combos on the periphery adds. For these situations Steel Cyclone is your best friend, which again generates way more hate than Flash, does damage, helps self heal, and doesn't interrupt combos.

If you prefer Flash because that's what you're used to, that's fine. Just remember it does have negative consequences for WAR. All I can say is try practicing with just Overpower because that's how you'll improve when pulling as WAR. Eventually, you'll find you don't need Flash.