The big man himself is being nominated for S-rank, but I'll have to disagree with that nomination right now. I say this as a very avid user of Reuniclus that incorporates it onto my teams very often, and I think that I have enough experience with this exceptional 'mon to have a say here.
There was a time when Reuniclus was still an incredibly underrated Pokemon, as it had somehow fallen to the depths of RU. People very quickly rediscovered this monster, realizing that with Magic Guard, great bulk, and good coverage, it could easily dismantle most Stall and Balance teams. Psyshock+Focus Blast offers great 2-move coverage on the Calm Mind set, and Reuniclus's naturally high special attack allowed it to have the upper hand in Calm Mind wars with anything not named Chandelure. Calm Mind dismantles stall, and the Trick Room set is very good at destroying offense thanks to Reuniclus's natural traits, most importantly its high speed under Trick Room. These sets allowed Reuniclus to once again make a name for itself in this tier, and it soon found itself propelled into the A-ranks as one of the most dangerous setup sweepers in the tier.
That was then, this is now. Balance and stall are not the same as they were back then, and they have adapted to specifically check Reuniclus. Reuniclus had an edge over the typical CroCune set, as it could set up to +6 on it. However, the main Suicune sets nowadays always incorporate Roar, and in the right circumstances they can stall Reuniclus out. A number of defensive answers to Reuniclus are getting more and more common, especially Doublade, Mandibuzz, and Jellicent, whose rise in viability has a lot to do with its exceptional ability to check Reuniclus. Even Quagsire has seen a slight rise in usage over this thing.
To top it all off, Reuniclus has always struggled against offense thanks to its low speed. This Pokemon's stellar bulk may allow it to check some things, but its Calm Mind set is hard pressed to sweep against offense. Get a little bit of chip damage on it, and Krookodile, Mega Beedrill and Hydreigon can beat it easily. Doublade, also commonly found on offense, resists Reuniclus's typical coverage moves and can use it as setup fodder. Chandelure, perhaps one of the tier's best wallbreakers with its Choice Specs set, aborts this fetus with ease. There are other checks, like Encore Whimsicott, Mega Blastoise, Feraligatr, Heracross, and Knock Off Mamoswine, all of which contribute to this set being hard to use against offense. Yes, I know that some of these threats can't switch into Reuniclus, but they can still either predict right on the switch or revenge kill this thing. The Trick Room set has the inverse problem, performing admirably against offense but faltering against Stall and Bulky balance, where common Pokemon like Mandibuzz and Florges can take hits from it and recover health.
To top it all off, Reuniclus has always struggled against offense thanks to its low speed. This Pokemon's stellar bulk may allow it to check some things, but its Calm Mind set is hard pressed to sweep against offense. Get a little bit of chip damage on it, and Krookodile, Mega Beedrill and Hydreigon can beat it easily. Doublade, also commonly found on offense, resists Reuniclus's typical coverage moves and can use it as setup fodder. Chandelure, perhaps one of the tier's best wallbreakers with its Choice Specs set, aborts this fetus with ease. There are other checks, like Encore Whimsicott, Mega Blastoise, Feraligatr, Heracross, and Knock Off Mamoswine, all of which contribute to this set being hard to use against offense. Yes, I know that some of these threats can't switch into Reuniclus, but they can still either predict right on the switch or revenge kill this thing. The Trick Room set has the inverse problem, performing admirably against offense but faltering against Stall and Bulky balance, where common Pokemon like Mandibuzz and Florges can take hits from it and recover health.
TLDR, I think that Reuniclus is an incredible Pokemon with great attributes all around, but I also think that S is too high for it. It's a terror for any team to face, but most good players know that nowadays and prepare accordingly. The fact that defensive teams are prepared for it and that some of the tier's key offensive threats can demolish its (arguably) best set means that this Pokemon ought to stay A+, at least for now.