EX is often appropriate, I think the trick is just figuring out when something is really uncalculatable. EX gets thrown around pretty casually for things you actually can break down into rankings or numbers when it is chiefly reserved in canon for things either so bizarre or so strong you can't rank them formally on the usual scale system. If you can articulate like, what a rank beneath it would do, then you probably shouldn't use EX.
For example, you can't down rank 'making the impossible possible' in Pioneer of the Stars or 'guarding literally everything' in Avalon. A rank below Avalon would have to be 'guards against almost everything'? That doesn't parse, so EX fits. ^^; Or in Army of the King's case, you can't even begin to calculate the power level of 2000 individual Servants, all of whom have variable stats.