FYI,Shubarugo(Brave/Adamant 252 Attack) can OHKO Bold 252 Hp/252 Def Reuniclus with a STAB Megahorn:
405 Atk vs 273 Def & 424 HP (120 Base Power): 384 - 452 (90.57% - 106.60%)
With a CB,it's guaranteed.
607 Atk vs 273 Def & 424 HP (120 Base Power): 576 - 678 (135.85% - 159.91%)
How much does Reuniclus do back?
A +1 Psycho Shock does: 430 Atk vs 246 Def & 281 HP (80 Base Power): 75 - 89 (26.69% - 31.67%)
A +1 Focus Blast does: 430 Atk vs 246 Def & 281 HP (120 Base Power): 151 - 178 (53.74% - 63.35%)
That's a Shubarugo with NO investment in its bulk(It's not like he wouldn't invest in it) and with a Brave nature and 0 Speed IVs,he will always be slower than Reuniclus and can "out speed" the TR variants.
With 252 HP,Reuniclus can barely 2HKO him: 430 Atk vs 246 Def & 344 HP (120 Base Power): 151 - 178 (43.90% - 51.74%)
Although he has coverage issues,Shubarugo is fantastic against Reuniclus
Spiritomb isn't really the only counter.
I'm expecting people to say: "Oh,it's not used much. It's crap. Ban reuniclus."
I'd also like to point out that Shubarago is excellent for Rain teams crushing things like Nattorei, Burungeru, Virijion, Rankurusu (and Trick Room), Tentacruel, Toxicroak, Parasect, Tyranitar, Abomasnow...Shubarago is really underrated and why it isn't used more is bizzare. I mean, it's a bug/steel with an attack even stronger than Scizor's and with higher defenses. Sure it doesn't get Bullet Punch but it carries the capability to crush many of rain's counters (like Rain needs any more help, shush, don't tell anyone in case it by some miracle survives). It's movepool is small but that's actually a good thing somewhat. You can afford to run Swords Dance/Iron Head/Megahorn/Filler and for a Rain team, you can use that filler slot to be...you guessed it, Rain Dance. Tyranitar is scared of Megahorn/Iron Head (although if it gets in safely and carries a fire move you're in trouble but otherwise...) and could be an excellent back up plan in case Politoad is dead or you need something to set up rain and can't afford to risk Politoad. It also cuts the fire weakness in half and with better special defense, it can take those random unstabbed Hp Fires or unstabbed Flamethrowers (except Tyranitar by virtue of switching weather and Ninetails although both risk severe potential damage switching in).
I wonder if some bizzaro Trick Room/Rain Dance team is possible with Shubarago and friends being how slow Swift Swimmers are without rain and with they outspeed and Trick Room does outspeed then you set up Rain and outspeed some more...
Anyhow, back to Rankurusu. Shubarago is an excellent counter (except for very random Hp Fire just like Scizor) although the Spiritomb/Burungeru example does not quite match Scizor/Shubarago.
Rankurusu is annoying, very, but probably not worthy of being nominated for Uber. Although then again, if people want to ban it, that's fine too because that is one less potential annoying threat and a glue of Trick Room.
If Rankurusu does happen to be banned, his lower stage prevo could replace him because it has quite similar defenses with evo stone (actually higher although I don't know how much the hp difference would make so probably about equal or only slightly weaker) and Magic Guard as well.