Legion is a charge item and shares the same cooldown with other charges like damage add, instant 1500 range DD etc. but does not share the same cooldown with pots.
This is an advantage for assassins, but to make it clear, advantage does NOT equal imbalanced.
Assassins will often have their opponent diseased (unless they use it in conjunction with purge), while only themselves being diseased in very rare cases, that alone means that a removal of the item would hurt assassins the most.
And in regards to IP+heal pot+heart vs. heal pot+Heart, and me being pedantic... In the later example, Heart will make up a greater percentage heal.
In short... While I use Heart on my thane, make no mistake removal of Heart will hurt non-IP users more than IP users, but assassins the most.