i
no it isn't easy to test, there's a ton a variables that need to be considered.
That shouldn't matter, because your option would cover both. We're talking about a short hop nair approach here.
Yes it does matter. Weak hit nair is much easier to cc that strong hit nair; both in a skill sense and a feasibility sense. Fox nair has weaker kb in the latter part of its duration, which makes cc a better option for when this happens.
What does this even mean? When Fox hits you, you react with Ftilt. That's it. If he's coming straight above you for some inexplicable reason, that's the only time this won't work (but what Fox would actually just fall on you with nair?), but chances are he'll be approaching on the same plane.. His running short hop plane or his short hop in place plane, which, CC ftilt covers both. It doesn't matter what point he hits you at because even at the lowest possible point of nair, 3 frame ftilt still beats shine.
The height of where nair hits you makes all the difference in how fast shine comes out relative to your reaction.
Let me give you a similar example.
When you nair shine ON SHIELD, making sure the nair hits the lower half of their shield is important. Why? Because the you have to consider how much shield stun nair does VS how many frames it takes for Fox to land, Lcancel, and do the next action (in this case is shine). You cannot simply ignore the amount of time it takes for Fox to land; this is important and can be the difference between getting shield grabbed vs being completely safe on shield.
Thus in the scenario of cc nair shine, how low the nair hits you can depend on the angle/distace that fox is jumping from, and how late the nair is pressed/what height was nair started.
Smash practice with zhu and cactus explained it pretty well, I'm going to try and find a link.
MK is a pretty short character, so you might not think this matters but it's not something you should overlook.
I come with the experience of CC grabbing nair shine with marth (which is way easier cause his grab range is huge), but I know CC grab works with mk... sometimes. Now you've got to consider how you can prevent sliding, how much distance you need to cover with dash grab, how much earlier boost grabbing will help, etc. It's really quite complicated, and I'm not convinced ftilt is that much simpler.
This is very relevant however. Obviously being at too high of a percent means your CC won't work anymore, or that you'll slide too far to follow up. At that point you need to play the ground game and threaten with your pokes, etc.
This is a 'duh' part of the cc.
My other point still stands that ftilt is a poor CC option based on the fact that fox can simply cc you back. (or ff then cc if you hit him in the air)
Edit: Making a vid doesn't really help, because unless you're running a setup similar to Kadano's stuff you're not going to convince me the fox isn't being frame perfect.