There is alot on full stall that can destroy CM Reuniclus. Taunt Deoxys-D, Taunt Mew, Celebi, Jirachi, Sableye (who isnt useless like Tomb this gen anymore woo) to name just a few and all are very good Pokemon otherwise. Full stall is still possible especially now Excadrill and Thundurus are banned who absolutely slaughtered defensive teams. Ive gotten very high on the ladder with it. Infact id say things like Sableye, NP Celebi and Mew are more annoying for full stall. Also you say Perish Song doesnt do the job when it does. The simple fact you've forced it out with it doing 0 damage to Celebi stops it sweeping your team, do this until its their last Poke (you rack up hazard damage each time its forced out on the rest of their team) and you get a free kill and thus a win.
Also CB Tar is a crap Reuniclus counter tbh. 1. It cant even guarantee a OHKO on Runiclus. 2. Reuniclus used Trick Room/Focus Blast in general.
Reuniclus imo fits the uber characteristics because its exactly like Salamence last gen though, not because it solely beats stall. It will cost you a mon if you guess wrong on the set its packing (not even mence could do that last gen as its sets had one common check - steels) and its CM checks are TOTALLY different from its TR sets bar Jira and Sp.def Scizor (maybe Sableye too). The TR set will crush 90% of the CM counters and really hurts offensive teams and utterly reverses momentum so in a way its detriment to both offense and defense with its sets.
Pokemon CAN and have been banned for making a play style unplayable. One of the main arguments for Exacrill's ban was that you couldnt play HO with it around (it also really hurt any team without Gliscor essentially and that extends to stall with its "un-blockable Rapid Spin" and sheer power but yeah). Yanmega was also banned in gen 4 UU because you couldnt play HO with it around.
Lastly with Ditto its not that hard to stop unless you set up 6 times KNOWING they have one (Team Preview ftw) in which case you lack brain cells lol. Ditto is a revenge killer as it cannot switch in due to its horrific defenses/hp so you just need to switch to resists if it comes in to revenge and you haven't moronically set up 6 times early game before its eliminated which is asking for a reverse sweep tbh. Ditto is also extremely easy to wear down with no lefties, SS and SR damage actually, plus if you actually expect it coming in to stop your sweep (read: suicide for the ditto user) just attack on the switch. On a positive note it stops noobs DDing 12 times. ;D
is good competitively I think but I didn't choose it as a favorite because of that, it just happened to work out quite nicely. :3
Alot of my favs are good competitively but i didnt pick them for that alone.
Azelf, Omastar, Victreebel, Qwilfish, Weezing and Cacturne are all really good in the lower tiers whilst Ferrothorn, Jellicent, Cloyster and Skarm are all excellent in OU.