Omnistone granting Hail of Blades makes it a bad keystone for potential ranged AP users.
If you're ranged AP, you absolutely do not want this Hail of Blades outside of potentially
(mantra CD)

This may seem like a good amount of users, but the truth is that all of them have much better choices than Omnistone. Certain avoidable (on Riot's part) Keystones that give absolutely no benefit prevent many potential users of the rune from ever being viable with it.
Start looking into these edge-cases of the rune and add a 'floor' for additional AS/AD if you hit a certain AP threshold, that way you only ever get the keystone if you'll actually make use of it. Early game would likely be excluded from this minimum requirement, as not enough AP would be built by this time.
This Keystone is going to go completely unused outside of a select few ranged champions if you can get effectively useless options out of it with its 8 second minimum ranged CD.
and it works great. I Play a tanky bard so electrocute by the mid game (~15min) it is not a strong as hitting peeps with three fast meeps.