Very good in ailment teams, as Pestilence will kill any enemy after 2 turns, no questions asked.
Perfect synergy with Nergal. Nergal helps to remove any barriers and ensures Plague's poison can land via Weak, which in turn triggers his own Meslamtaea passive.
Very effective as a random PVP defense to catch teams without ailment cleanse.
Decent magic stat means lets him deal damage while his targets are incapacitated.
Cons
Ailments generally fall flat in late/endgame PvE because of inflated enemy stats
Hard countered by Ailment Cleansers like Daisoujou or Kinmamon in PVP.
Pestilence only kicks in at the end of enemy's turn, giving them one whole turn to cleanse it away or just outright kill him.
Requires a specific team built around him to really work.