Its not a problem with instant abilities... its a problem with animation lock. This is rooted in every single ability, like trying to cast Spirit's Within (silence) during a Rage of Halone animation. From what I understand, this was a design decision purely for aesthetics so you don't have animations overlapping or overwriting making your character do funky things. Its unfortunately because, of course, the gameplay suffers a lot for it. Benediction and Hallowed Ground being really strong examples of how animation lock causes unecessary and frustrating wipes.

Also notice how every dps rotation has to go GCD - Instant - GCD - Instant in its rotation. Its because you literally cannot fit more than one instant ability between rotations. I think its technically bad wording on their part to even call these abilities instant when they are, in fact, not even close to it.