Its based purely on hit, which means multi hit and AoE have insanely high chances to proc devotions. Shadow Strike, for instance, has upwards of 4 instances of damage it does on a single target, Dual wield hits twice with it, and Nightfall hits twice if dual wielding as well, as well as hitting twice everything within a 5m radius with Nightfall. Combo that with the fact it has a 3.8 second base CD, which adds roughly 50-70% proc rate for devotions, it has a godlike devotion proc chance, to the point of being near 100% chance regardless of the base proc rate of the devotion itself.
Just remember that a lot of devotion abilities and some item procs have cooldowns. An aoe that hits 15 times on the first tick doesn’t matter past the first success if the skill its triggering has a 5 second cooldown. It just makes it more likely that it will trigger.
I have to disagree on that. Maybe it used to suck, but given that each Wind Devil can proc its own cooldown of a Devotion, it is one of the better skills in the game for devotion procs. My Druid rather consistently has 6 Leviathan Whirlpools out at full ramp.