It is procedural in the sense that there's always a case of the week but PoI is much more serialised than most procedurals imo. It's more like Fringe, there are always ongoing mytharc storylines and in every single episode there's some sort of development with lasting effects (or a flashback or an answer to a mystery etc). Also, there are multiple major arcs, each with its own separate villain, so it doesn't get stale.

Like, you know how on H50 (which I've quit watching too btw) they have Wo Fat? Imagine a show with 4/5 Wo Fats. And imagine a show that doesn't "forget" about its villains/major storylines to have random totally standalone episodes, only bringing the villains back during sweeps. There's always continuity, to a degree that's unusual for a procedural imo.