[@Yankee] A little bit of column A and a little bit of column B. My understanding is hat EX means a sort of exception value that can't be contained by the usual ranking system. That means that it's classified like that because it's weird and doesn't play well with the ranks at some level, but at the same time, if something's able to be classified like that, it's probably going to be pretty up there as opposed to being equal to an E rank ability. 

I'd recommend staying away from EX parameters unless you think it's really well-justified, but I'll be much more open to EX skills and, to a slightly lesser extent, Noble Phantasms.