smash is all gimmicks and game knowledge
an example of this is there's this thing that Reflex likes to do with Wario where he nairs into people's shield, nair stays out a long time and has very little lag but its still not really safe to do that. but people underestimate how long the move stays out and how little the lag is. So he rarely gets punished for it. in fact sometimes people drop their shield before the moves over. this **** works on good players I've seen Calvin and lou fall for it plenty of times. at first, I was like AWWW YEAA REFLEX MINDGAMMEEEZZ. but no that's just the opponent either falling into a gimmick or not knowing the game as much as Reflex. this was confirmed when i saw Reflex play M2K at ausom i think it was. M2K did not fall for that **** once.
Depends on the character.
Wario N-Air is just a mix-up, but it is relatively safe in that. It's basically a sex kick that hits twice.
It has good range (for Wario) and ends very late in the animation. As a result, you can hit a shield with the first hit, and then you hover nearby with the rest of the animation. People freeze because they don't want to get hit with the second hit, which can combo into jab, grab, F-Smash, and Waft, among other things.
Meta Knight's N-Air comes out on frame 3 (tied for the second fastest aerial in the game, the fastest being Meta Knight's U-Air, but trying to buffer a U-Air from a jump makes you U-Smash, and Meta Knight players don't want to bother with things like timing), which is why Meta Knight can punish it after taking the first hit via shield. Peach can do it. Luigi can do it. Sheik can do it. Other characters have invincible Up-Bs that can do it, like Meta Knight (again), Marth, Samus, and Mario.
With the frame 3 aerials, you pretty much have to see it coming or really have a reason to N-Air. It's a pretty safe option for Peach and Luigi, but since neither character's approach game is very good, and because their N-Airs last a decent while, they can't just throw out a N-Air and expect not to get punished. Meta Knight can. Same with the Up-B, since Mario and Marth would get punished badly on a whiff, but Wario cannot (I repeat, CANNOT) reasonably punish a properly-canceled Shuttle Loop without a super-hard read.
I do happen to know a lot about the game, probably more than Calvin or Lounis. Calvin doesn't quite understand Meta Knight's ability to shut out Wario's moves, and Lounis's character can't do it nearly as easily. N-Air is, generally-speaking, a very safe move when used intelligently. Meta Knight just breaks the game.