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Player -0 and I were talking about Wario in the Social Thread but I figured it'd be more suited to being discussed here.
I also don't really think shoulder bash needs to have a disjoint on it, but shielding it is an effective counter if you're any good at OoS stuff (I'm not...). Obviously I haven't tested this, but pretty much any character should be able to WD OoS after shoulder bash on shield and get a grab on Wario before he escapes his fairly massive endlag. The pushback there isn't really a problem anyway - regardless of how easy it is to punish him OoS, he has a mixup in that he can just jump the shoulder bash over your shield if he sees that you're going to do that, and most characters can't punish that unless they have a fast, disjointed usmash, since jumping OoS would probably get you hit by him as he passes over, WDing to get something probably isn't fast/far enough, and most characters don't have something that's fast and far-reaching enough to jump after he's passed and still hit him as he goes by.
It's punishable, but he gets mixups. The mixup is
good, but still not unstoppable. I think shoulder bash is mostly fine because there's a good bit of interaction and available counterplay and mixups to fight the counterplay, which means it's still typically a good experience.
That said, Wario really should rarely be using shoulder bash at a time where there's a chance he'll hit your shield anyway - it works well as a read or a tech chase, or sometimes as a combo finisher, but it's not something he should rely on much in neutral in most circumstances, I think.