The Heroic Ability Legacy unlock doesn't allow you to pick any ability you like; that would indeed lead to some balancing nightmares.
There was very careful consideration over which abilities to allow as unlocks. With the limitations of the ability only being usable during your Heroic Moment (requiring an active Companion) and the cooldown on that, you shouldn't see too much 'Heroic Ability spam'. We see the unlock as a fun, limited ability that many players will enjoy. It wasn't designed to unbalance the game.
Yup, all good.
I'm glad Bioware opted to implement this via Heroic Moment. Kudos.
Still, I could use it in PvP on a regular world map. But at that point, none of that is really game breaking since that sort of combat has no rewards, just bragging rights.