I think they are if they include the whole of the game. When I say the whole Im talking about the pocket, the object ball and the contact point. The vitality is proven when a system connects all of those things and enables a player to understand allowances that are to be made with spin.
Of course if someone asks a great player and how he does what he does and he says something like "I use the edge of the ball to tell me what to do," that means something to him but not to the mediocre player who wants to know what to look at in order to see where he needs to go.
But..if there is an example/ system that gets the player closer and you say...look at this and keeping looking for this and you will begin to understand what I see when I shoot shots......that is a little more concrete and at least the newbie has something to go on and the system in question is given more vitality.
I don't know of such a statement being made by any well known player about any of the known systems which puts the vitality of the ones we know about in question.
Can you be systematic without using a system?
Lou Figueroa