we just had an almost TPK because of a ghoul in a cramped L-shaped hallway, paralyzed the fighter, knocked out the alchemist, witch, and the wizard but finally got it with our flame sorcerer.. that was a bad roll night
As a DM/GM in mostly home brews I have used Npc characters to fill in a role that the party is missing, most often clerics and the odd wizard. Sometimes they are characters I played in the past but most often they are just Npcs that I am excited to try out as well. I know when I run the Emerald Spire for my group I am adding the Iconic Warpriest as an npc to help them out with healing. Their group consists of a brawler half orc, swashbuckling gnome, slayer human, and an investigator dwarf.
The first Society adventure I played, Master of the fallen fortress, was terrible because the GM added his magus that was a level higher than us and out shown the Halfling bone oracle and the dwarf rogue.
As long as the npc does not steal the show or become more important than the players it should be fine. it is all about balance and making sure everyone is having fun