Reuniclus is currently A+ in the SS RU Viability Rankings. Just a look at the rankings would tell how it is viable in the SS RU metagame.
Reuniclus's high HP stat and amazing abilities in Magic Guard and Regenerator allow it to function as a bulky setup sweeper or a specially defensive pivot. With high Special Attack, it's no offensive slouch either, so it can use Psyshock to come out on top in Calm Mind wars against Pokemon such as Sigilyph and Rest variants of Suicune.
Reuniclus is a premier defensive Pokémon in the SS RU metagame which can also pose to be a threatening offensive threat with Calm Mind + Recover. Magic Guard allows Reuniclus to not get worn down by Toxic, hazards and other chip damage, whereas Regenerator + Assault Vest gives it the famed RegenVest combination, which is not very common in the legal metagames (that is, Regenerator Pokémon can sometimes not utilise Assault Vest to its full extent, like Ho-Oh).
Building around Reuniclus is also relatively easy, as it does not require much support outside Knock Off absorbers and hazard setters, which you'll be running on almost every team anyways. A lot of other Pokémon also benefit from Reuniclus' capability to squeeze moves and exert pressure in the form of Future Sight and Calm Mind on the opposing team.
Finally, here is a replay between TPP and Confide from the ongoing RUPL. Confide used Reuniclus to its full extent, with Future Sight and Knock Off both coming in clutch. TPP also used a Reuniclus but sadly it did not do anything much notable in the match.
Hope it helped.