Kangaskhan also has a flaw yet it is S-Rank, and Carracosta isn't entirely flawless either, and both need support to function. Kangaskhan needs something that removes things like Gurdurr, while SmashCosta is fairly desperate for a free switch.Yeah, so that practically means that it needs some form of "support" to be used effectively. It would probably be S-Rank if it can utilize a recovery item, but foregoing Eviolite kinda sucks. Metang is also a candidate for being worn down by repeated Focus Blast, U-turn, and Megahorn faster than other Pokemon (at least Missy has Pain Split). Metang more times than not has been proven to be vital for my team's success, but unfortunately, it has flaws that prevents it from being S-Rank.
It also says in the S-Rank description that the S-Rankers are supposed to have "numerous positive traits with little-to-no flaws" and tbh Metang doesn't fit that. :(
But yeah, all Pokémon in NU have a flaw, one way or another, or they would not be NU.