So yes it is fair to compare spells from different classes.
If ypur goal is to compare classes power then sure ...
If your goal is compare spells of single class to determine if people will use any other or only zhe strongest one ... whike playing that class ... then im sory but its missleading.
I think that before you can say "X is OP", you're going to have to provide a frame of reference. If it's an overall class evaluation, then you have to provide the class(es) that are outshone by X. If it's a spell, then you need to have something to compare it to. It cannot be OP in a vacuum, it has to be OP compared to something else.
