I've never been a fan of this UX technique:
It's almost always counterintuitive (deliberate cognitive dissonance) because a single scrolling motion produces varying scrolling effects
Many sites nowadays (particularly mobile interfaces) have multiple layers of interactions with drawers, drop downs, slide-ins and dialogs, and parallax scrolling adds additional unnecessary Z-axis complexity to interfaces.
Proponents often cite the "realistic" interaction of foreground versus background elements, but ultimately the screen is a flat environment so faking deep 3D perspective is incredibly hard to get right without incurring confusion over what the actual perspective is. In real life, every parallax site I've seen forces a change in the perspective point from the user, which is the opposite of realistic.
Many (even most) parallax interfaces fail to properly indicate which elements are in the parallax background, so it's distracting to users when pieces start moving in different proportions.
Google's Material Design framework has some nice principles explaining why content shouldn't collide in the same 2D space, and parallax websites often violate this physical principle.
I can see why parallax effects took off originally as a gimmicky creative technique, but now that the novelty has worn off, is it time for this effect to die off or are there still valid uses for it aside from perhaps the very simplest long page websites with hero images?