I think you can to an extent. There's no blanket rule of course. But I'm fairly sure history is literred with great coaches that weren't good at every club they coahced.
Pagan?
Blight?
Ayres?
So, to be sure, you are saying great coaches can be limited by the players they are working with?
If so, totally agree. My observation is far to much value is placed purely on the level of succes a coach has rather than on a ratio of their success with some sort of factor that recognises the ability of the team they inherited. Of coure it is far easier and less subjective to evaluate a coach purely on ladder position rather than include an evaluation of the team in general. I think coaches should be evaluated on what the quality of the list they leave behind is, not simply the success during their period of coaching.