Now let's talk about Suicune late-game cleaning capacities with Calm Mind. Suicune is very bulky on the physical side and can boost his SpD with Calm Mind. His only way to heal is Rest (+ Sleep Talk), what makes him very luck reliant like I said before.
Even if Suicune is extremely bulky, powerful physical attackers can still beat him with Atk boost (Mega Scizor, Bisharp, DD Mega Altaria, Charizard X, SD Garchomp, Breloom...). On the special side, powerful special attackers can break him at +1 : LO Thundurus, Mega Gardevoir, Mega Manectric, Raikou, TG Energy Ball Manaphy. That's not all, a mono offensive water type makes him easily walled by Grass-types like Mega Venusaur, Amoonguss, Celebi, Breloom. Suicune is also completely shut down by Taunt, so Taunt Users like Gengar, Mega Sableye and Mew easily beat it. Encore users also shut down Suicune, so Mega Alakazam and even Politoed make him very vulnerable. I don't have spoken about Rest + Sleep Talk which make Suicune quite easy to break in fact because you only need to be able to score a 3HKO to beat Suicune (Suicune sleeps during 2 turns).
Suicune is by no way a bad Pokemon but defensively he has only a few niches over Slowbro (being able to wall CB Scizor and Bulky SD Mega Scizor, Scarf Tyranitar, Weavile and CM Mega Sableye) and is extremely Scald burn reliant for stuff like Bisharp ; and luck (only 30% chance) isn't a reliable way to wall threats. His late-game cleaning capacities are quite limited and he's really too much luck reliant so definitely not B+ material.