Why is NIN considered OP or highly viable?

[Oscura]

Trick attack is essentially an AoE buff like Embolden, Battle Litany, and Brotherhood. In fact, it's better than all three. You could argue it's better than Litany and Brotherhood combined, especially because of it's incredibly low cooldown. In high levels of play threat manipulation and management is actually super important, a NIN being in the party during Heavensward basically guaranteed that tanks hardly had to generate any hate or make any effort to. As it stands right now, NIN also does incredibly well with personal DPS and while it is getting nerfed it probably won't negatively influence it too much.

Although yes, Trick Attack is the main reason.