I think this mastery is designed for AP/AD nukers.
However, currently, it is used not only for them but also for all other champs which can activate the mastery easily.
It's not a surprising thing that we have 5 Thunderlord's in a team.
So my suggestion is: make it like Stormraider's Surge. (instead of nerf the mastery directly or nerf lots of champs which get benefits from this.)
Then, it wouldn't be a dominant mastery.