Serperior's comparable to regular Diancie. It sounds good on paper but it has so many counters and checks that it's not viable.
...That's gotta be the worst analogy I've ever seen. The two are nothing alike. Plus, by that same logic, Tyranitar has so many counters and checks that it's not viable.
Diancie's problem is that with most of its niches, Diancie finds it horribly outclassed at them, beyond being a Trick Room lead which is a poor niche regardless. Being so slow and bulky with average offenses, even things that would make sense like Calm Mind sets are outclassed entirely by Mega Slowbro and Mega Sableye. No reliable recovery outside of Leftovers does it no favors either. Not to mention its low speed and numerous weaknesses leave it checked by a LOT of things, including but not limited to Scizor, Mega Metagross, Lando-T, pretty much every bulky water type, etc.
Serperior on the other hand, while definitely having some noteworthy flaws that will hold it back from being top tier, is the only viable user of Contrary, and has a fantastic speed tier in the OU metagame. With Greninja gone, it literally outspeeds every unboosted Pokemon in the tier except Talonflame (whose priority makes that irrelevant anyway), Mega Lopunny, and Mega Manectric, the latter two of whom need a turn to Mega Evolve. It definitely has some Pokemon it struggles to break through, such as Heatran, Mega Venusaur, Talonflame, Mega Altaria, and Mega Charizard Y... But the rest of the tier can find themselves having trouble switching in if they're careless, because a base 130 power STAB with a free Nasty Plot followed up by the appropriate coverage move can screw over a lot of things. Contrary Leaf Storm is practically a no-risk move. Even things that resist it, such as Mega Metagross, Scizor, Ferrothorn, Latias, Dragonite, etc. do not want to eat the +2 HP Fire/Dragon Pulse that follows it.
No one expects Serperior to be a top tier OU threat, but it at least has niches in OU. It can threaten defensive teams by accumulating +2 boosts with every Leaf Storm it spams, and it's solid speed tier can be a nuisance to offense as well. True, its base SpA is initially low, and grass typing always carries with it some unfortunate weaknesses, but to say it's not viable at all... well, that's just a little short-sighted.
And again, it's nowhere near comparable to Diancie because Diancie's niches can be done by other Pokemon, while Serperior's the only Pokemon in the game who can get free +2 boosts by spamming its strongest STAB attack.