Shulk is a character that is deliberately held back by a lot of factors. Terrible frame data, bad inertia, sub-par recovery, etc. He's clearly designed with the idea that Monado Arts are a serious game changer for him. It's been discussed how much of a game breaker Monado Arts would be on any character other than Shulk. His entire gameplan is based on being in the right Art in the right time and negating his inherent weaknesses. You can't discount his Monado Arts as they make him.
Vanilla Shulk can only really approach with nair and fair. Once in Jump and Speed he can use his amazing pivot grab, pivot f-tilt, and bair, he gains mix-ups with SHFF aerial or pivot grab or pivot f-tilt or tomahawk grab. Additionally, his bad inertia is completely eliminated as he now has the best aerial mobility in the game. Once in Buster, his moves start becoming safer on shield and he gets ridiculous damage off of relatively easy confirms. Once is Shield, he can take advantage of a larger and more hp, faster regnerating shield and build up rage for a incredibly strong counter. Once in Smash, his long range makes him ridiculously scary as all of his tilts, aerials and smashes will now kill in the 90-100% range.
Shulk is definitely an odd character (seriously, what is up with that utilt) with crap frame data, but it's not difficult to see how he can be good, at least not in bottom 5. There's been a lot of tech discovered for him, canceling landing lag with Monado Arts, buffer art cancels. Will he ever be high tier? If Zero starts wrecking with him, then he'll certainly be in people's minds.
And yes, nair is amazing. Auto cancel, long lasting hit box, leads into grabs, leads into combos, covers entirely around Shulk, covers almost every ledge option, and deals a decent 8%.