Except rifle nades don't have an obvious animation, or timer and are much more accessible than demos. There's also a bug that can be abused to trigger the animation after the nade fires.
The animation is obvious and even when there's no animation - because relic can't fix such bugs - there's still the firing sound + travel time.
The fact remains: you can know where a rifle nade is going to be used (at least when Grens haven't reached vet 2 yet) because you clearly see them and it takes time for the nade to hit its target.
Demos, once planted, can be triggered immediately, resulting in a squad wipe.
Demos are more expensive but also more effective in return. You can wipe a maxim with rifle nades if it's clumped up but you can also recover it (if you supported it).
Demos that wipe a squad Grens, Obers, Sturm or even Pios have inflicted way more MP bleed.