For what?
If any part of your enemies' massive hit-rings intersects the already massive conal AoE, you get a hit. Even with small enemies pushed out and towards your immediate flanks by larger enemies, there is no chance of missing an enemy with your conal AoE when using it correctly.
The radial AoE's only unique advantage is QoL. And you have to sacrifice range for that.
No, it doesn't. You can just jump and lock your camera, just like with any other instant skill used when running away.1. Doing backwards Overpower pulls effectively requires non-default controls.
As a targeted skill, the direction of your animation is irrelevant. The cone will always be centered on your target.
You'd only ever need to move back and forth for Mythril if there would otherwise be more than 10 yalms' distance between the edges of the two most distant mobs' hit-rings. Such is incredibly rare.2. Having to reposition every GCD when spamming Overpower into Mythril so you can hit everything is just clunky. If you need to reposition to group up mobs better, you can do that on every other tank with circle AoEs just fine.
Remember, AoEs do not strike enemy models; they strike their hit-rings, which are far, far larger. AoEs do not have to intersect the middle of those hit rings, either, only any part of them.
Barely. A 8-yalm 120-degree conal AoE has an area of ~67 sq. yalms. A 5-yalm radial AoE has an area of ~78 sq. yalms. The difference is negligible relative to some quarter-seconds' movement in any given direction. Any cone wider than 140 degrees would already match it for total area.3. Circle has a larger total area.
And that's only been the case since mid-Stormblood, when all radial AoE sizes were increased in response to people complaining about radial AoEs being harder to hit all enemies with than conals. (Note: Linear AoEs were widened at the same time; until then, conals were the most reliable and easiest to use AoEs.)
For a single straggler, you'd use Provoke regardless (or, previously, you'd have used it to gather them into Overpower's formerly greater effective area of effect and thus have no stragglers).4. If you need to pick up stragglers, just use Voke or Tomahawk, though you'll likely have fewer stragglers to begin with because 3.
The difference is merely that my AoE now has same range as enemy auto-attacks, meaning that if I want to hit multiple stragglers in the same GCD, I must get hit. So instead of just positioning halfway decently for a tool that could do all this in a single GCD, I'd now have to spend further GCDs or be hit.