It's a legitimate complaint given how long the animation is and how you have to wait nearly 2 seconds from when you use it for it to have any effect. In any content where you need to get AoE enmity quickly rather than having 5-10 seconds of time to set up a pull, Overpower is effectively worthless because of that insanely long animation time. WAR is effectively *required* to have Flash for T4, which is kind of wrong when you consider that WAR *should* be considered to have a perfectly serviceable CD on its own.
The *only* advantages that Overpower has are its damage and better enmity per GCD. Flash has better secondary effects, better activation speed, better resource cost, larger and more useful area of effect (PbAoE is better than cone), can be used without risking breaking CC, and doesn't require a target so that you can actually precast it as something is spawning rather than having to wait until something already has. Considering we're talking tank DPS here, the damage advantage of Overpower is negligible in the confines of a group, especially when you stack it against the outrageous TP cost that prevents you from using it more than a few times before having to stop in order to recoup TP.
The delay on Overpower is a legitimate concern (so is the delay on Brutal Swing) since there's so much stacked *against* Overpower compared to Flash that it could be considered a balance concern that needs to be addressed at some point. It's not a *major* issue since it hasn't rendered WAR explicitly weaker than PLD (largely because WAR can get Flash via additionals when it's absolutely *required*), like the scaling of WAR self heals or the whole kerfuffle involving Wrath stacking benefits and their consumption, but it's not something that should simply be dismissed.
I'd be quite happy to see the animation of Overpower sped up by double so that I'm waiting for it to land only slightly longer than Flash, but I wouldn't be completely let down if it the devs ignored it since it doesn't really complicate my life much and it's hard to *prove* that it makes Overpower explicitly worse than Flash because the two are balanced asymmetrically.