its really not very hard to get it consistent frame perfect. here's how:
first practice doing a SH oos. this sounds stupid, but practice it till you never FJ.
now if you are having problems getting from x->a or y->a, USE Z. seriously. there are more buttons. you can put a different finger on it. practice doing a frame 1 nair (not oos) by standing next to a puff and trying to hit her with a sh nair on the "way up". if you get it frame 1, it will hit her as you are rising. later frame, it will hit once you have started decending.
now put it all together by practicing going from being in shield to the nair. once you can do it against a standing puff, then practice against a CPU that will do pressure (look at teh 20XX training pack for more info on this)
i promise, you can do frame perfect nair oos with a bit of practice. its a 1-frame-link, BUT NO JOHNS
seriously though, a lot of people ive helped with this cant get their fingers from x/y -> a fast enough, but as soon as i made them use Z, they were doing it. Z ftw